V ol. 253: 3 9 -5 4 , 2003
MARINE ECOLOGY PROGRESS SERIES
Mar Ecol Prog Ser
P u b lish ed M ay 15
Terrestrial deposits on intertidal sandflats:
sediment characteristics as indicators of habitat
suitability for recolonising macrofauna
Vonda Cummings1*, Simon Thrush1, Judi Hewitt1, Ali Norkko1,2, Stuart Pickmere1
'N ational Institute of Water and Atmospheric Research, PO Box 11-115, Hamilton, N ew Zealand
2P resent address: Department of Marine Ecology, G öteborg University, Kristineberg Marine Research Station,
450 34 Fiskebäckskil, Sw eden
ABSTRACT: E levated rates of sedim entation as a result of hum an activities is a recognised problem in
m any m arine environm ents. Thus, it is im portant to develop a m echanistic understanding of the im pact
and su b seq u en t recovery of terrestrial sedim ent deposits in these areas. This pap er describes an e x ­
perim ent to investigate possible reasons for the slow recovery of intertidal soft-sedim ent m acrofaunal
communities following sm othering by storm -associated terrestrial sedim ent deposits. We m easured, and
m onitored changes in, a large num ber of physical and biogeochem ical properties of terrestrially derived
sedim ents, in order to identify those characteristics that w ere most im portant to potential colonists. Prop­
erties of th e terrestrial sedim ents w ere rem arkably different to those of the surrounding sandflat and
show ed little signs of change over the duration (4.5 mo) of the experim ent. Despite this, w e w ere able to
identify that tem poral changes in m acrofaunal community composition w ere strongly correlated with the
levels of chi a, total carbohydrate, phosphorus (P), nitrogen (N) and coarse sand in the sedim ents. We
recom m end that future studies of m acrofaunal recolonisation following deposition of terrestrial sedim ent
should include m easu rem en t of these sedim ent properties. In addition, analyses w hich isolate the
biologically available portions of carbohydrate, P and N m ay provide even m ore insight as to the
im portance of these properties as settlem ent cues and indicators of recovery. We also assessed the
influence on m acrofaunal com m unities of the conditioning the sedim ent receives prior to being d e ­
posited on th e sandflat (i.e. m ixing w ith seaw ater or freshw ater), and found no effect of this on the im ­
pact on existing m acrofaunal com m unities or on the subsequent recovery of sedim ents or m acrofauna.
KEY WORDS: Sedim entation • Intertidal soft-sedim ents • Terrestrial sedim ent deposits • M acrofauna •
Recolonisation • Sedim ent characteristics
---------------------------------- Resale or republication not perm itted without written consent of the p ublisher----------------------------------
Elevated rates of sedim entation in near-shore m arine
environm ents are of global concern, an d several
studies have indicated the bro ad scale degrad ation of
valuable habitats d u e to m assive inputs of sedim ent
(McKnight 1969, Peterson 1985, Lundin & Linden 1993,
GESAMP 1994, G ray 1997, Ellis et al. 2000). Deposits
from individual sedim entation events have been
d etected several cm d eep and, especially in areas sh el­
tered from w ind-w ave disturbance, they can be persis­
tent an d result in long-term deg rad atio n an d alteration
of the h ab itat d u e to sm othering of infauna, increased
turbidity and changes in sedim ent grain size. H ow ­
ever, there is presently no clear m echanistic u n d e r­
standing of w hy these deposits rem ain so 'unattractive'
to colonists long after their introduction to the m arine
environm ent. H ere, w e describe the properties of te r­
restrially derived sedim ents deposited in an intertidal,
soft-sedim ent habitat, in order to identify those ch arac­
teristics th at are most im portant to potential colonists.
The role of hum an activities in elevating the input of
terrestrially derived sedim ent to m arine areas has b een
recognised for decades and is w ell docum ented (e.g.
M eade 1969, M illiman & M eade 1983, Shaffer & Parks
1994, C undy et al. 1998). For exam ple, th ere are n u ­
*Email: v.cummings@ niwa.co.nz
© Inter-R esearch 2003 • w w w .int-res.com
INTRODUCTION
40
M ar Ecol Prog Ser 253: 3 9 -5 4 , 2003
m erous records of elev ated sedim ent loads to w a te r­
w ays resulting from catchm ent developm ent, defor­
estation an d road building activities (e.g. M egahan &
Kidd 1972, Doeg & Koehn 1990, F ahey & C oker 1992,
Davies & N elson 1993, G rayson et al. 1993). Fine sed i­
m ents w hich are rem oved from the land in this w ay
m ay find their w ay to coastal areas w ithin hours/
m inutes (Fahey & C oker 1992), particularly in associa­
tion w ith flood events (Nichols 1977, W heatcroft et al.
1997, W heatcroft 2000). O n m ixing w ith seaw ater, fine
terrestrial sedim ents flocculate and can settle onto the
seafloor; their accum ulation on the seafloor can ad v ers­
ely affect th e resident benthic com m unities (Johnston
et al. 1981, Fahey & C oker 1992). In storm -associated
sedim entation, terrestrial sedim ent m ay be w ashed into
a catchm ent stream and tran sp o rted dow nstream to the
estuary, after w hich it m ay be deposited directly onto
the sandflat, or rem ain in suspension for a tim e prior to
deposition. Thus, the sedim ent can be m ixed to a v ary ­
ing d e g re e w ith seaw ater and freshw ater, and the
fluidity of the deposits will also vary.
In m arine soft-sedim ent habitats in N ew Zealand,
catastrophic inputs of terrestrial sedim ent associated
w ith storm events often occurs on such a scale that
large areas can be sm othered an d essentially, defaun ated (McKnight 1969, Jo hnston et al. 1981, Foster &
C arter 1997, Burd et al. 2000). For exam ple, following
a storm even t in th e W hangapoua E stuary in 1995, a
10 cm d eep deposit of terrestrial sedim ent covered an
entire sandflat. This resu lted in sm othering of the resi­
dent fauna, including extensive shellfish an d seagrass
beds, an d th e su b seq u en t recovery of th e m acrofaunal
com m unities took several years (authors' unpubl. obs.).
Slow recovery has also b een noted following other n a t­
urally occurring large-scale deposition events (Foster &
C arter 1997, Burd et al. 2000). In an experim ent d e ­
signed to mimic the deposition of terrestrial sedim ents
on an intertidal sandflat, N orkko et al. (2002) m onitored
the response and subsequent recovery of the m acrofau­
nal com m unity for 13 mo. The terrestrial sedim ents in ­
du ced hypoxia an d anoxia on the sandflat, killed all
fauna resid en t u n d er the deposits, affected biogeo­
chem ical fluxes, an d essentially altered the habitat.
While recovery w as still incom plete at th e en d of the
experim ent, N orkko et al. (2002) w ere able to illustrate
that rem obilisation of th e terrestrial sedim ents via bioturbation by crabs, an d transport of am bient sedim ents
on to th e surface of th e deposits w ith bedload, w ere
im portant facilitators of m acrobenthic recovery.
T errestrial sedim ents that are d eposited during c a ta ­
strophic events differ from m arine sedim ents in term s
of their physical (e.g. grain size, penetrability) and bio­
geochem ical (e.g. m icrobial composition, nitrogen [N]
and phosphorus [P] content, nutritive value to benthos)
composition. They frequently contain a larg e propor­
tion of silt and clay-sized particles, are enriched w ith
organic m atter, and levels of high-quality food (esti­
m ated by chi a content) are significantly d ep leted
(Norkko et al. 2002). Once deposited on a sandflat, their
properties will be further modified. For exam ple, w hen
deposition results in sm othering of infauna, oxygen is
dep leted and levels of hydrogen sulphide are elevated
(e.g. similar effect to the sm othering associated w ith
algal mats; H ansen & K ristensen 1997, Tallqvist 2001,
Fetzer et al. 2002). As organism s will respond to such
factors in different ways, they will obviously influence
the potential for recolonisation of the deposits by
m acrofauna and ultimately, their recovery.
The im portance of other biogeochem ical properties,
such as levels of am m onia, biofilms and m icrophyto­
benthos, as cues of habitat suitability for potential
colonists is being increasingly recognised (e.g. Flem ­
m ing & D elafontaine 2000). Studies by W oodin et al.
(1998) and M arinelli & Woodin (2002) have show n that
gradients of oxygen and am m onia in near-surface (top
few mm) sedim ents are altered by short-term (h), smallscale disturbances, such as those caused by natural sed ­
im ent erosion and deposition, and th at individual or­
ganism s can detect these changes. Properties of biofilms
that accum ulate on estuarine sedim ent surfaces can also
provide inform ation on habitat suitability. Biofilms are a
product of com plex assem blages of algal diatom s and
bacterial com m unities and contain, am ong other
things, carbohydrate and protein. C hanges in quantities
of these substances in surface sedim ents may reflect the
breakdow n of these exudates by m icrobes and the su b ­
sequent release of nutrients into the pore-w ater for use
by m icrophytobenthos. T here is a strong, positive co rre­
lation betw een the am ount of m icrophytobenthos and
colloidal carbohydrate presen t and the stability/erodability of the sedim ent (e.g. S utherland et al. 1998).
V aluable additional inform ation can be gain ed from
estim ating the portions of these biofilms that are bio­
logically available to the fauna as a high-quality food
source, rath er th an just p resenting bulk values (Mayer
et al. 1986, 1995, S utherland et al. 1998). However,
these m ethods are m ore tim e-consum ing, expensive,
and require a num ber of replicates for each sam ple
collected, essentially lim iting the num ber and types of
sedim ent properties th at can be realistically m easured
in a study w ith lim ited resources. Thus, m easuring
num erous bulk properties is a good approach tow ards
initially isolating potentially im portant properties,
w hich can subsequently be studied in m ore detail.
This p a p er describes the results of an experim ent
designed to investigate possible reasons for the slow re ­
covery of m acrofaunal com m unities following sm oth­
ering by terrestrial sedim ent deposits. The experim ent
w as designed to m easure and m onitor (1) tem poral
changes in biological, chem ical and physical properties
Cum m ings et al.: Terrestrial sedim ent deposits in m arine soft-sedim ent habitats
of the sedim ent, as w ell as (2) m acrofaunal recolonisa­
tion, for several m onths following defaunation, and
(3) to identify possible cause-effect relationships
b etw een these. Lastly, as the sedim ent m ay en ter the
estuary in a nu m b er of w ays in association w ith a storm
event (e.g. mixing w ith seaw ater or freshw ater), w e
also investigated w h eth er the m acrofaunal recovery
differed w ith the 'conditioning' the terrestrial sedim ent
receives prior to its deposition on the sandflat.
MATERIALS AND METHODS
Study area. The study w as conducted on an in te r­
tidal sandflat in the W hangapoua Estuary, on the e a st­
ern side of C orom andel Peninsula, N orth Island, New
Z ealand (Fig. 1A,B). The experim ental site w as situ­
ated at about m id-tide level. The sandflat sedim ents
are m ainly sands (fine sand 30.29% volume, m edium
sand 60.76% an d coarse sand 8.58%) w ith very little
silt (0.36%) an d no clay.
The estuary is surro u n d ed by steep, m ainly forested
catchm ent on 3 sides, and is open to th e sea on the
north w estern side. The catchm ent soils are com prised
m ainly of m oderately to strongly leach ed central
brow n g ran u lar clays (Orbell 1973).
Experimental treatments and set-up. We identified 3
scenarios for storm -associated sedim entation.
50:50 ireshw ater:terrestrial sedim ent: Terrestrial
sedim ent is w ash ed into a catchm ent stream , tra n s­
ported dow nstream to the estuary an d deposited
im m ediately on th e sandflat. Therefore, the sedim ent
is m ixed mostly w ith freshw ater prior to deposition.
This treatm en t will be referred to as 'F50'.
N orth Island
Whangapoua Estuary
W hangapoua
E stuary
"
km
S50 S70
F50 S50
F50
50 m
F50
S70
C
S70 S50
To channel
Fig. 1. (A) Whangapoua Estuary, North Island, New Zealand.
(B) The study site (•) in the Whangapoua Estuary. (C) The
arrangement of the experimental treatments in each block
(1 to 3)
41
50:50 seaw ater:terrestrlal sedim ent: Terrestrial sed ­
im ent is w ashed into a catchm ent stream , transported
dow nstream to the estuary, and rem ains in suspension
for a tim e prior to being deposited onto the sandflat.
Therefore, the sedim ent is m ixed m ostly w ith seaw ater
prior to deposition. This treatm ent will be referred to as
'S50'.
As the terrestrial sedim ent en tering the estuary does
so most likely w h en the tide is covering the sandflat,
both the F50 and S50 treatm ents w ere ad ded during a
high tide period.
70:30 seaw ater:terrestrial sedim ent: This treatm ent
w as included to determ ine the effect of a m ore fluid
seaw ater: sedim ent mix. It w as a d d ed w h en the san d ­
flat w as exposed during a low tide period, and will be
referred to as 'S70'.
All 3 terrestrial sedim ent treatm ents resulted in
10 cm d eep deposits in the 1 m in diam eter experim en­
tal plots. A control treatm ent w ith no terrestrial sedi­
m ent add ed w as also included. T reatm ents w ere rep li­
cated 3 times, and replicates of each treatm en t w ere
arran g ed in a random ised block design (Fig. 1C).
Blocks w ere arran g ed parallel w ith the adjacent c h a n ­
nel, w ith 50 m betw een each block. Individual re p ­
licates w ithin a block w ere sep arated by 10 m. The
sedim ent-w ater m ixtures referred to are w eight
percentages.
Experimental set-up. The terrestrial sedim ent used
in the experim ent w as collected from the W hangapoua
catchm ent, in an a rea of forest being cleared for road
building. At source this sub-surface sedim ent w as
com prised of 2.76% clay, 38.50% silt, 56.02% fine
sand and 2.72% m edium sand. After excavation, the
sedim ent w as coarsely sieved (4 cm mesh), loaded into
concrete m ixer trucks and freshw ater or saltw ater w as
add ed to create a slurry of know n concentration.
Freshw ater for m ixing w as pum ped into the m ixer from
a nearby stream , w hile saltw ater w as pum ped from the
estuary. The slurries w ere m ixed overnight. The next
day, the appropriate slurry for each treatm ent w as
loaded into a m onsoon bucket, delivered to the estuary
via helicopter, and deposited into the circular ex p eri­
m ental plots on the sandflat.
The F50 and S50 plots w ere set up on February 3,
1999, w hile the S70 and control plots w ere set up on
February 4, 1999. The F50 and S50 treatm ent plots
w ere p rep are d by inserting a plastic ring around the
plot perim eter w h en the sandflat w as uncovered by the
tide. The ring pro tru d ed 1.8 m above the sedim ent sur­
face. As the tide covered the sandflat the rings g ra d u ­
ally filled w ith seaw ater, and at high tide the top of the
rings w ere still above the w ater level, preventin g any
loss of the mix. O nce high tide w as reached, the mix
w as deposited into the rings. The rings w ere kept in
place until the w ater h a d drain ed from the sandflat,
42
M ar Ecol Prog Ser 253: 3 9 -5 4 , 2003
and w ere th en carefully rem oved. The S70 an d control
treatm en t plots w ere p re p a re d by inserting a m etal
ring into the sedim ent so that 10 cm protruded. The
clay w as deposited into th e S70 treatm en t rings w hen
the sandflat w as uncovered by th e tide. The rings sur­
rounding the S70 an d control plots w ere rem oved
before th e next tide covered th e sandflat.
Sampling. Plots w ere sam pled for m acrofauna and a
variety of sedim ent characteristics over the 4.5 mo
(127 d) following th e terrestrial sedim ent deposition.
The exact dates of sam pling and the sam ples collected
on each occasion are given in Table 1. Sam pling w ithin
the plot w as done in a rotational m anner, to en sure that
sam ples w ere not collected from a previously disturbed
position. Sam ples w ere collected at least 30 cm from
the plot p erim eter to avoid e d g e effects, an d core holes
w ere filled w ith d efau n ated sandflat sedim ent to avoid
confounding effects of the plot sedim ents slum ping
into th e holes.
C olonisation: C ore sam ples w ere collected to m oni­
tor colonisation of the plots by settlem ent of new
recruits an d post-settlem ent stages (>250 pm; h e re ­
after 'colonising m acrofauna'), an d also to assess the
response of larg er m acrofauna (>500 pm; h ereafter
'm acrofauna'). Colonising m acrofauna sam ples w ere
collected on 7 occasions. On each date, a core sam ple
(5 cm in diam eter, 2 cm deep) w as taken, p reserv ed in
70% isopropyl alcohol an d stained w ith rose bengal.
The anim als retain ed on a 250 pm sieve w ere later
sorted, identified and counted. Larger m acrofauna
w ere sam pled prior to sedim ent deposition (Day -2),
and on 2 occasions afterw ards (Days 6 and 127), using
a 10 cm in diam eter, 15 cm deep corer.
S e d im e n t characteristics: Several small sedim ent
cores (each 2 cm in diam eter, 2 cm deep) w ere col­
lected to assess sedim ent characteristics. Two cores
w ere collected and pooled for chi a analysis, 1 core for
analysis of am m oniacal-nitrogen (NH4-N) in the porew ater, and 3 cores w ere pooled and later sub-sam pled
for analysis of total m icrobial biomass, total carbohy­
drate, total protein, sedim ent P, sedim ent N and % p a r­
ticulate carbon (% C). Surface sedim ent scrapes w ere
collected and assessed for grain size and organic con­
tent. All sedim ent sam ples w ere im m ediately frozen
and stored until they could be analysed.
Sedim ent for chi a analysis w as extracted in 95%
ethanol, and the extract w as processed using a spec­
trophotom eter. An acidification step w as used to se p a ­
rate degradation products from chi a (Sartory 1982).
The sedim ent core for porew ater NH4-N determ ination
w as centrifuged (3000 rpm, 15 min), and the porew ater
carefully d ecan ted and analysed using the autom ated
p h en ate m ethod (APHA 1998).
Total m icrobial biom ass (pg g_1 dry wt) w as m e a ­
sured by dichlorom ethane extraction of the w et sedi­
m ent followed by p ersulphate determ ination of the
phospholipid (Findlay et al. 1989). Total
carbohydrates w ere estim ated using
Table 1. D ates of each sam pling occasion, nu m b er of days since th e experim ent
w as set u p and th e sam ples collected on each day. The terrestrial sedim ents
the phenol-sulphuric acid m ethod (Liu
w ere deposited on F ebruary 3 (F50 an d S50 treatm ents) an d 4 (S70 treatm ent),
et al. 1973) w hich m easures both the
1999. Sam pling days in bold are those u sed to denote each sam pling occasion
sugars and polysaccharides present.
throughout th e paper. 'S edim ent p ro p erties' include th e following: % coarse
Sedim ent (50 to 100 m g dry wt) w as
sand, m edium sand, fine sand, silt an d clay, penetrability, sh ear stress, carbo­
hydrate, protein, m icrobial biom ass, chi a, phaeophytin, p o rew ater N H 4-N,
dispersed in 1 ml of deionised w ater
sedim ent P, N, % C an d organic content
w ith 10% phenol and sulphuric acid.
The absorbance w as rea d in a Shim adzu UV-1601 UV-Visible spec­
Sam pling date
Days since set-up
Sam ples collected
(1999)
F50, S50
S70, control
trophotom eter at 485 nm. Following
correction for a sedim ent-sulphuric
2 Feb
-1
-2
M acrofauna
acid blank, carbohydrate content w as
9 Feb
6
5
Sedim ent properties,
expressed
as glucose equivalents (mg
colonising m acrofauna,
g_1 dry wt). The phenol-sulphuric acid
m acrofauna
carbohydrate analysis estim ates the
16 Feb
13
12
Sedim ent properties,
colonising m acrofauna
total sugars (pentoses and hexoses)
26 Feb
23
22
Sedim ent properties,
presen t in the biological fraction, but
colonising m acrofauna
does not distinguish b etw een o rg an ­
11 M ar
36
35
Sedim ent properties,
isms or exudates. Total protein w as
colonising m acrofauna
assayed by a m odified Lowry m ethod
25 M ar
50
49
Sedim ent properties,
(H artree 1972). Sedim ent (30 to 50 mg
colonising m acrofauna
dry wt) w as hom ogenised in 1 ml
7 M ay
93
92
Sedim ent properties,
colonising m acrofauna
deionised w ater and the absorbance
11 Ju n
127
126
Sedim ent properties,
read at 650 nm on a Shim adzu UV-1601
colonising m acrofauna,
UV-Visible spectrophotom eter. The
m acrofauna
results w ere expressed as BSA equiva-
Cum m ings et al.: Terrestrial sedim ent deposits in m arine soft-sedim ent habitats
lents (mg g_1 dry wt). The Lowry protein m ethod used
in our study m ay lead to overestim ates of protein con­
tent d u e to th e p resen ce of phenolic com pounds. H ow ­
ever, it is a relatively sim ple m ethod suited to analys­
ing large num bers of sam ples.
Sedim ent P and N (mg g_1 dry wt) w ere determ ined
by Kjeldahl digestion at 300°C followed by ammonium -nitrogen an d pho sp h ate determ ination using a
Technicon AAII autoanalyser. Total % C in the sed i­
m ent w as m easu red using high tem p eratu re com bus­
tion in a Perkin Elm er 2400 CHN analyser. Control
sam ples w ere tre a te d w ith 5 M HCl prior to analysis to
rem ove any carbonate (mainly shell pieces) present.
G rain size w as d eterm in ed by digesting th e sed i­
m ents in 6% hydrogen peroxide for 48 h to rem ove
organic m atter, an d dispersion using C algon. A G alai
particle analyser (Galai Cis - 100; G alai Productions)
w as th en used to calculate % volum es for the coarse,
m edium an d fine sand, silt an d clay fractions. O rganic
content, m easu red as loss on ignition (LOI), w as
estim ated by drying th e sedim ent at 90°C for 48 h,
th en com busting in a muffle furnace for 5.5 h at
400°C. The d ep th of the clay layer w as also m easured,
along w ith the sedim ent shear stren g th (using a pocket
shear vane) an d penetrability (using a pocket p e n e ­
trom eter).
Statistical analyses. Univariate analyses: U nivariate
analyses w ere conducted to d eterm ine differences
betw een experim ental treatm en ts over time, in sed i­
m ent properties an d count d a ta (from the m acrofauna
and th e colonising m acrofauna cores). C ount data
analysed w ere either total nu m b er of individuals, total
num ber of taxa, or ab u n d an t taxa (i.e. taxa occurring
on av erage > 1 individual p er core in any plots on the
majority of sam pling occasions). Burrow ing crabs
(Helice crassa, M acrophthalm us hirtipes an d H e m i­
grapsus crenulatus) w ere com bined in all analyses. For
2 tax a from th e colonising m acrofaunal count d a ta that
w ere analysed for treatm en t effects, non-convergence
w as attain ed using g en eralised linear models. Thus,
for these 2 variables, a ran k transform ation w as used
and an ANOVA th en carried out on the ranks.
Before differences b etw een treatm en ts w ere investi­
gated, d ata w ere check ed for norm ality (Shapiro-W ilk
test) an d hom ogeneity of variances (Cochran's test).
Sedim ent d ata b ased on p ercen tag es w as arcsinetransform ed before testing. W here sedim ent d ata did
not m eet ANOVA assum ptions for norm ality and hom o­
geneity of variances, log transform ations w ere used.
W hen count d ata did not m eet these assum ptions, g e n ­
eralised linear m odelling techniques, w ith a Poisson
error structure an d a log-link function, w ere used.
O n the few occasions w h ere such d ata w ere over­
dispersed, quasi-likelihood techniques w ere used.
D ifferences b etw een experim ental treatm ents over
43
tim e w ere investigated using treatm ent and tim e as
categorical factors and including a Time x T reatm ent
interaction term . Significant m ain effects (p < 0.05)
w ere investigated by using m ultiple contrast sta te­
m ents. W hen a significant interaction term (p < 0.10)
w as found, differences betw een treatm ents w ere
investigated for each sam pling occasion separately.
M ultivariate analyses: Colonising m acrofaunal
count d ata w ere H ellinger-transform ed before princi­
pal com ponent analysis (PCA) of the raw d ata w as con­
ducted, following L egendre & G allagher (2001). To
investigate the relationship betw een the colonising
m acrofauna and sedim ent properties, redundancy
analysis (RDA) w as conducted on all treatm ents, and
th en on control treatm ents only. Due to analytical
problem s, the following d ata w ere g e n era ted to enable
a full suite of sedim ent properties to be included in the
RDA: (1) NH4-N could not be d eterm ined for the con­
trol plots on Day 50, due to insufficient porew ater vol­
um es. Therefore, this w as estim ated for each control
plot replicate using an average of the values recorded
on Days 13, 23, 36 and 93. (2) Penetrom eter and shear
vane m easurem ents w ere not m ade in the control plots
on Days 6, 13 or 23. Thus, an average of the values
recorded from Days 36, 50, 93 and 127 w as calculated
for each replicate control plot, and these values w ere
used for each m issing sam pling occasion. (3)% C v a l­
ues w ere not available on Day 127. Therefore, the re la ­
tionship b etw een % C and carbohydrate w as d e te r­
m ined, and the resultant equation used to g en erate
corresponding values for % C at Day 127 (R = 0.84).
The RDAs identified a num ber of highly correlated
sedim ent properties, especially % clay content. For­
w ard selection w as conducted to determ ine the im por­
tant sedim ent variables (canonical correspondence
analysis; ter Braak 1986, 1987), avoiding choosing
highly correlated variables, such that the final m odel
did not contain variance inflation factors >10. Forw ard
selection w as rep ea ted a num ber of times, varying the
order of variable selection to en sure that the m odel
selected w as stable.
RESULTS
General observations
The terrestrial sedim ent dep th gradually d ecreased
over the experim ent, from an initial d ep th of 9 to 10 cm
im m ediately after deposition, to around 4 to 6 cm
4.5 mo later (Day 127). T errestrial sedim ent d ep th w as
quite variable w ithin a particular plot, due to the rip ­
ples on the sandflat surface below the deposits. T here
w ere no obvious differences in d ep th b etw een the dif­
ferent treatm ents. On the first few sam pling dates, the
44
M ar Ecol Prog Ser 253: 3 9 -5 4 , 2003
S70 plots a p p eared relatively m ore 'sm ooth' and fluid
th an th e other sedim ent treatm ents. All terrestrial
sedim ent treatm en t plots rem ain ed distinct from the
surrounding sedim ents throughout th e experim ent. In
the initial w eeks of th e experim ent, th e w eath er w as
calm an d sunny, so th e sedim ent 'set' and dried out
on the sandflat, an d cracks a p p e a re d in all plots.
A lthough the plots stayed p roud of th e surrounding
sandflat, as tim e progressed their diam eters shrank due
to build-up of m arine sedim ents at th e plot edges.
Unlike a previous experim ent (Norkko et al. 2002),
they did not becom e covered by sedim ent m oving with
the bedload, nor w ere they ero d ed by w ave action. The
only visible chan g e in th e terrestrial sedim ent plots
w as the ap p earan ce of num erous tiny burrow s in the
clay layers on Day 23 (up to 142 burrow s 0.1 n r 2),
caused by a larg e influx of juvenile crabs. As the
experim ent progressed, these burrow s becam e larger
as th e crabs g rew in size. Conditions throughout the
experim ent w ere calm, w ith very little w ind-w ave d is­
turbance along w ith very little rainfall (authors' obs.).
M acrofauna (^500 pm)
The sandflat sedim ents w ere d om inated by spionid
polychaetes (Aquilaspio aucklandica, A o n id es o xy­
cephala, Prionospio sp.) as w ell as th e n ep th y d A g la o ­
p h a m u s an d nereids. The bivalves N ucula hartvigiana,
A u stro ven u s stutchburyi an d M acom ona liliana w ere
also common. Prior to sedim ent deposition (Day -2) the
num ber of individuals found w as similar in each plot,
as w ere the nu m b er of taxa (Fig. 2). However, by Day
6, num bers in the terrestrial sedim ent treatm ents had
declined m arkedly, an d w ere considerably low er than
those in the control plots. Sam ples ta k e n from te rre s­
trial sedim ent plots in a com panion study show ed that
all m acrofauna u n d er the deposits w ere d ead after 8 d
(Hewitt et al. in press). This com panion study b eg an on
the sam e day as our experim ent, and used terrestrial
sedim ents identical to those u sed in our S70 treatm ent
(Hewitt et al. in press). Even by the en d of the ex p eri­
m ent, 127 d later, the ab u n d an ce an d diversity of
m acrofauna in the sedim ent treatm en t plots w as still
significantly less th a n in the controls (p < 0.0001;
A ppendix 1). T here w ere no differences in m acrofau­
nal ab u n d an ce or diversity b etw een the 3 terrestrial
sedim ent treatm ents.
C olonising macrofauna (^250 pm)
The total num ber of individuals an d the total num ber
of taxa w ere significantly higher in the control plots than
the terrestrial sedim ent treatm en t plots throughout the
experim ent (p < 0.0001; A ppendix 1, Fig. 3). T here w ere
no differences in ab undance of individuals or taxa b e ­
tw een terrestrial sedim ent treatm ents.
The bivalve N ucula hartvigiana and the polychaete
L um brinereis brevicirra (<1 ind. core-1) w ere found in
the control plots only, although in low num bers (0 to 2
and <1 ind. core-1, respectively). Conversely, bu rro w ­
ing crabs (M acrophthalm us hirtipes and H elice crassa)
w ere only found in the terrestrial sedim ent treatm ents.
Two taxa (Aquilaspio aucklandica and n ereid poly­
chaetes) w ere found in both the control and terrestrial
sedim ent treatm ents (Fig. 4). N either taxon exhibited a
significant Time x T reatm ent interaction (p > 0.15), and
both w ere significantly m ore abu n d an t in the control
plots (p < 0.0001; A ppendix 1).
Fig. 5 illustrates the change in colonising m acrofau­
nal com m unity com position in the different treatm ents
over the 127 d experim ent in 2 dim ensions. This PC A
ordination explained 67 % of the variability in com m u­
nity com position (first 4 axes), w ith 2/3 of this variabil­
ity explained by Axes 1 and 2 (Fig. 5). T here is a dis­
tinct separation betw een the control plot and sedim ent
treatm ent com m unities throughout the experim ent
(Fig. 5). W hile the control plot com m unities are very
similar over the experim ental period, the com m unities
80-,
(A) Number of individuals
Control
- a - S70
■S50
60-
a>
oo
40-
0
20-
tu
CO
d± n
2
0
_Q
E
D
5_
§
10
-.
6
127
(B) Number of taxa
s
2
6
127
Days
Fig. 2. N um ber of (A) m acrofaunal (>500 pm) individuals and
(B) tax a found in each experim ental treatm en t at th e b e g in ­
n in g an d en d of th e experim ent. N um bers p resen ted are
m eans (±SE) p er core
Cum m ings et al.: Terrestrial sedim ent deposits in m arine soft-sedim ent habitats
(A) N u m b er of individuals
(A) Aquilaspio aucklandica
—0— Control
•
1-50
—a — S70
- S50
40-
45
—o— Control
•
F50
- a - S70
- -■ - - S50
15-1
12-
30-
2
O
20-
O
o
o
a>
a>
CL
CL
nr
co
+L
6 13
E
Ë
23
36
50
93
127
m-
36
50
36
50
93
127
(B) N u m b er of ta x a
93
127
1U-,
3
CB
®
i-'---*6 13 23
(B) N ereids
c
s
nr
co
+L
a>
«
8_
2
1
IL
0
6 13
23
36
50
93
127
D ay s
6 13 23
D ays
Fig. 3. N um ber of (A) colonising m acrofaunal (>250 pm) in d i­
viduals an d (B) tax a found in each experim ental treatm en t
throughout th e experim ent. N um bers p re se n ted are m eans
(±SE) p e r core
Fig. 4. N um ber of individuals of 2 taxa, (A) A quilaspio a u ck­
landica an d (B) n ereids, found in each experim ental tre a t­
m ent thro u g h o u t th e experim ent. N um bers p re se n ted are
m eans (±SE) p e r core
of all 3 sedim ent treatm ents exhibit large, variable
ch anges in composition. On 2 occasions, the S70 com ­
m unity m oved n e a re r to that of the control in the o rd i­
nation space; how ever, on n eith er occasion w ere they
close to the control com m unity of that p articular sam ­
pling occasion (Fig. 5), and the com m unities w ere still
distinctly different (see also Figs. 3 & 4).
The relative instability of th e terrestrial sedim ent
com m unities is further illustrated by exam ining the
total nu m b er of 'n ew ' taxa colonising the plots (i.e. sp e ­
cies that w ere not found in th e plots on the previous
sam pling occasion) and the total num b er of taxa that
w ere p resen t on the previous d ate (Fig. 6). The control
plots clearly have h ig h er num bers of th e sam e taxa
p resen t from one sam pling occasion to the next. Inter­
estingly, th ere are also a larg e num b er of new taxa
found in th e control plots, illustrating th e transient
n atu re of some tax a found on this sandflat.
but th ere w as no consistent distinction b etw een the
properties of the 3 terrestrial sedim ent treatm en ts (with
the exception of protein). The terrestrial sedim ents
have consistently low er levels of porew ater NH4-N,
coarse and m edium sand, chi a and chi a:phaeophytin
ratio, and higher levels of carbohydrate, protein, N, P,
% C, % organic content, fine sand, silt, clay and shear
strength relative to the control treatm en t sedim ents.
M icrobial biomass, phaeophytin and sedim ent p e n e tra ­
bility w ere the only properties m easured w hich did not
show a clear distinction betw een controls and sedim ent
treatm ents throughout the experim ent. These sedim ent
properties are discussed m ore fully below. Details of
the results of the statistical analysis of each sedim ent
property are p resen ted in A ppendix 2. C:N and
C :carbohydrate ratios w ere also investigated, but as
neither proved to be a useful diagnostic tool for distin­
guishing betw een treatm ents, these are not presen ted
here.
Sediment properties
G rain size
The various sedim ent p roperties m easu red generally
show a clear difference b etw een th e control an d te rre s­
trial sedim ent treatm en ts throughout th e experim ent,
Throughout the experim ent, the control plot sedi­
m ents w ere com prised predom inantly of m edium (27 to
M ar Ecol Prog Ser 253: 3 9 -5 4 , 2003
2% )
46
- o - Control
— F50
-D -S 7 0
-■ -S 5 0
A xis2(19
0.8 -
0.4-
-0 .4 -
-
0.8
-
-
0.8
0
-0.4
0.8
0.4
A xis 1 (24.3% )
Fig. 5. H ellinger-transform ed colonising m acrofaunal com­
m unity d ata in each experim ental treatm en t on each sam ­
pling occasion, pro d u ced b y principal com ponent analysis.
Points are connected in tim e sequence. The % variance
explained by each axis is also given
Number of taxa
t p Ii Ï
Total carbohydrate, total protein and
total m icrobial biom ass
S70
S50
Number of taxa
8
6
4
2
0
■2
-4
-6
■8
6
13
23
36
Day
50
93
127
Penetrability and shear strength
The de-w atering of the terrestrial sedim ent tre a t­
m ent plots over tim e is reflected in the m easurem ents
of sedim ent penetrability and shear strength
(Fig. 7F,G). As the plots dried out and hardened, the
sedim ents req u ired m ore force to p en etrate and d is­
lodge sedim ents th an w as req u ired for the control plot
sedim ents.
F50
Control
66%) and fine (22 to 62%) sand, w ith 6 to 11 % coarse
sand and a very small fraction of silt (0.24 to 0.60%).
T here w as no clay in the control plots on any sam pling
date. The am ount of fine and m edium sand changed
gradually over the course (4.5 mo) of the experim ent,
from around 22 % fine sand and 66 % m edium sand on
Day 6, to 62% and 27% on Day 127, respectively
(Fig. 7).
In contrast, the terrestrial sedim ent treatm ents w ere
generally com prised m ainly of fine sand (40 to 80%)
w ith some silt (60 to 20% ). These treatm ents contained
a small am ount of clay (<9%) and no coarse sand
(Fig. 7). The % silt and % clay d ecreased m arkedly
over the course of the experim ent, and the % fine sand
increased accordingly (Fig. 7C -E). From Day 23
onw ards, clay levels in the terrestrial sedim ent tre a t­
m ents had d ropped such that some w ere no longer
significantly different from the control sedim ents
(Fig. 7E).
13
23
36
50
93
127
Day
□ New ta x a
■ Taxa also p re s e n t on previous sam pling d a te
Fig. 6. Total num b er of 'n ew ' tax a colonising th e plots (i.e.
species th at w ere not found in th e plots on th e previous sam ­
pling occasion) an d th e total n u m b er of tax a th at w ere also
p resen t on the previous date, in each experim ental tre a t­
m en t on each sam pling occasion
The carbohydrate and protein contents of the te rre s­
trial sedim ents w ere significantly higher th an in the
control plot sedim ents on all sam pling dates (p <
0.0001; Fig. 8A,B). In addition, th ere w as significantly
m ore protein in the S70 sedim ents th an in the F50
sedim ents throughout the experim ent. As th ere w ere
substantial am ounts of total carbohydrate associated
w ith protein in our analyses, the interference by
phenolic com pounds (sensu M ayer et al. 1986) is likely
to be low. The m icrobial biom ass ran g ed from 0.1 to
2.2 pg g_1 over the experim ent (Fig. 8C). W hile there
w as no distinction in m icrobial biom ass betw een tre a t­
m ents (p = 0.8044), the ANOVA did detect a significant
effect of tim e (p < 0.0001; Fig. 8C).
Chi a and phaeophytin
Chi a w as 2.3 to 13.0 times low er in the terrestrial
sedim ent treatm ents th an in the control plot sedim ents
C um m ings et al.: Terrestrial sedim ent deposits in m arin e soft-sedim ent habitats
15n
(A) % Coarse sand
(B) % Medium sand
47
(C) % Fine sand
100-
Mean (±SE)
8 0 -I
60 40200
0-
-
6 13 23
36
50
93
(E) % Clay
(D) % Silt
60-
Mean (±SE)
6 13 23
36
50
93
(F) Penetrability (kg/cm2)
3-
5040-
2
-
1
-
3020100
0-
-
6 13 23
36
50
93
(G) Shear strength (kg/cm2)
Mean (±SE)
—o— Control
—• — F50
-D -S 7 0
--■ --S 50
Fig. 7. C hanges in physical sedim ent p roperties of each treatm en t over
th e experim ent
(p < 0.0001; Fig. 8D). A lthough levels in creased in all
terrestrial sedim ent treatm en ts from Day 36 onw ards,
they h a d still not re ach ed those of the control sed i­
m ents by th e e n d of th e experim ent (Day 127). The
level of p h a e o p h y ta , a deg rad atio n product of chi a,
w as very low (<1.5 p g g_1) an d generally similar in all
treatm ents throughout the experim ent, w ith the ex cep ­
tion of the control plot sedim ents on Day 127 (Fig. 8E).
C onsequently, the ratio of chi a to p h aeophytin w as
alw ays significantly low er in th e terrestrial sedim ent
plots (p < 0.0001; Fig. 8F).
Porew ater N H 4-N, sedim ent P an d N
The level of NH4-N in p orew ater w as significantly
low er in the terrestrial sedim ent treatm ents th an the
control plot sedim ents on all but Day 93 (p < 0.0001;
Fig. 9A). Conversely, sedim ent P an d N w ere al­
w ays h igher in the terrestrial sedim ent treatm ents
(p < 0.0001; Fig. 9B,C). The tem poral fluctuations in
sedim ent P content m irrored those for sedim ent N in all
treatm ents.
% C and organic content (LOI)
T here w as m ore carbon in the terrestrial sedim ent
treatm ents th an the control sedim ents throughout the
experim ent (p < 0.0001; Fig. 9D). The organic content
w as also generally higher in the terrestrial sedim ent
treatm ents during the experim ent, w ith the exception
of Day 23, w hen th ere w ere no significant differences
betw een any of the treatm ents (even though the levels
in the control w ere still very low; Fig. 9E). A lthough
differences w ere found betw een the organic contents
of the terrestrial sedim ent treatm ents, these w ere not
consistent over time. O n Day 6, the S50 sedim ents had
a slightly higher organic content th an the F50 sedi­
m ents, and on Day 36, both of these treatm ents had
higher organic contents th an the S70 sedim ents.
C olonising macrofaunal community and
sedim ent properties
In the RDA th at included all 4 experim ental tre a t­
m ents, 4 sedim ent properties (% coarse sand, carbohy-
48
8
M ar Ecol Prog Ser 253: 3 9 -5 4 , 2003
A. Carbohydrate (mg g~1)
B. Protein (mg g~1)
Control
^ C. Microbial biomass (pg/g)
Mean (±SE)
4
6
4
2
*
0
Mean (±SE)
12
6 13 23
36
Í-
50
D. Chlorophyll a (pg/g)
^
2 E. Phaeophytin (pg/g)
F. Chlorophyll a : Phaeophytin
10
8
6
4
2
0
Days
Days
Fig. 8. C hanges in biological sedim ent pro p erties of each treatm en t over th e experim ent
drate, phosphorus and penetrability) explained 29.5%
of th e variability in the colonising m acrofaunal com m u­
nities b etw een treatm ents. The terrestrial sedim ent
treatm ents w ere associated w ith hig h er levels of carbo­
hydrate and P an d low er penetrability, w hile the control
plot com m unities h ad a strong, positive association w ith
coarse sand. T here w as no coarse sand in any of the te r­
restrial sedim ent treatm en ts (see Fig. 7). As noted for
the PC A (Fig. 5), th ere w as distinct separation betw een
the control an d terrestrial sedim ent com m unities
throughout the experim ent, and m uch higher tem poral
variability in the com m unities of all 3 of the latter tre a t­
m ents. This RDA explained a reasonable am ount of the
variability in com m unity com position betw een tre a t­
m ents, despite the low ab undance and sporadic occur­
rence of colonising m acrofauna and g eneral lack of
recovery in the terrestrial sedim ent treatm ents. This
result w as driven by the large differences in the sedi­
m ent characteristics betw een the terrestrial sedim ent
and control treatm ents throughout the experim ent.
(B) P h o sp h o ru s (mg g_1)
(D) % C arbon
(E) % O rganic c o n te n t (LOI)
(C) N itrogen (mg g-1)
Mean (±SE)
Mean (±SE)
(A) P o re w ater NH„-N (mg/l)
Contro
o—
— I—
-------O-------- o-------------------------- o
I-------- 1--------------1------------1---------------------------------------1----------------------------- 1
6 13 23
36
50
93
Days
127
Days
Fig. 9. C hanges in chem ical sedim ent p roperties of each treatm en t over th e experim ent. LOI: loss on ignition
Cum m ings et al.: Terrestrial sedim ent deposits in m arine soft-sedim ent habitats
______ 1
______ 1
______ 1
______ ______ i______ i______ i______ i______ i______
Recovery of sedim ents and macrofauna
\ N itro g e
l
i
l
i
C h lo ro p h y ll a A
i
-0.8
i
-0.4
i
0
0.4
0.8
49
1.2
Axis 1 (37.5%)
Fig. 10. H ellinger-transform ed colonising m acrofaunal Com­
m unity d ata in th e control treatm en t on each sam pling occa­
sion, and th e sedim ent properties im portant in explaining th e
variability in th ese com m unities, p ro d u ced by red u n d an cy
analysis. Points are connected in tim e sequence. The % v ari­
ance explained by each axis is also given
Thus, to help b etter identify sedim ent p roperties that
are im portant to colonising m acrofauna, w e conducted
an additional RDA using the control treatm ent data
only. T hree properties, chi a, P an d N, ex plained a very
large am ount (76.4%) of the variability in the control
com m unities during the experim ent (p = 0.0080;
Fig. 10). The im portance of these 3 properties is to
some extent reflected in th e sum m ary com m unity m ea­
sures of num bers of individuals and taxa. The most
im portant variable, chi a, in creased over the ex peri­
m ent, an d w as associated w ith a slight increase in
num bers of taxa (see Figs. 2B & 8D). H igher levels of
P an d N te n d e d to have a negative influence on
the ab u n d an ces of individuals and taxa (see Figs. 2 &
9B,C).
DISCUSSION
The m ain aim of this experim ent w as to characterise
the ch an g e in th e properties of terrestrial sedim ent
deposits on an intertidal sandflat, w hich occur during
storm -related sedim entation events, in association
w ith recolonisation of the deposits by m acrofauna.
M any different sedim ent characteristics w ere m ea ­
sured during the 4.5 mo experim ent, all of w hich could
be expected to evoke a response in potential colonists.
By relatin g ch anges in these properties to the m acro­
faunal com m unity in a m ultivariate way, w e w ere able
to isolate characteristics w hich are im portant as indica­
tors of h ab itat suitability to m acrofauna.
N um bers of colonists rem ained very low in the te r­
restrial sedim ents throughout the experim ent (i.e.
m ean ab u ndance <4 ind., <2 species core-1; Fig. 3),
and the sam e taxa w ere not generally found from
1 sam pling occasion to the next (Fig. 6). As this ex p e r­
im ent sp anned m ore th an 4 mo and encom passed a
period of high recruitm ent (authors' pers. obs.), it
is unlikely th at recruit lim itation contributed to this
pattern. For exam ple, average num bers of the spionid
polychaete A quilaspio aucklandica w ere reasonably
high in the control plots throughout the experim ent,
but very few settled in the terrestrial sedim ents
(Fig. 4). The only taxa th at increased in the terrestrial
sedim ents during the experim ent w ere n ereid poly­
chaetes (Fig. 4). Interestingly, nereids are relatively
tolerant of reduced levels of oxygen (Norkko 1998)
and are know n to modify nutrient fluxes in m uddy
sedim ents through their burrow ing activities (H ansen
& K ristensen 1997, M ortim er et al. 1999). This su g ­
gests they m ay play an im portant role in rem obilisa­
tion of the nutrients bound to the terrestrial sedim ents
and thus the recovery of the deposits in the longer
term.
W hile the results of our previous experim ents su g ­
gested th at m acrofaunal recovery w ould not occur
w ithin this short (4.5 mo) experim ental period (Norkko
et al. 2002, H ew itt et al. in press, T hrush et al. in press),
w e h ad expected to see m ore signs of change in the
sedim ent characteristics. M ost of the properties e x ­
hibited clear differences betw een the control plots and
the terrestrial sedim ents. T errestrial sedim ents had
consistently low er porew ater N H 4-N, coarse and m e ­
dium sand, chi a and chi a: phaeophytin ratios, and
higher levels of carbohydrate, protein, N, P, % C,
organic content, fine sand, silt, clay and shear strength,
relative to the control treatm ent sedim ents. M icrobial
biomass, phaeophytin and sedim ent penetrability
w ere the only properties m easured that did not show
consistent differences b etw een the control and te rre s­
trial sedim ent treatm ents.
The controls in this experim ent are essentially the
natural sandflat, w here the benthic com m unity is
already w ell established; thus w e did not test for the
effect of addition of m arine sedim ent to the sandflat.
However, w e do not believe this w ould account for the
significant differences in either the physical properties
or the colonists presen t betw een the terrestrial sedi­
m ent and control treatm ents. The reasons for this are
2 -fold: (1) previous w ork has illustrated that escape of
resident infauna sm othered by 'like' sedim ent (i.e. sim ­
ilar to that they are currently living in) is m uch m ore
likely th an if they are sm othered by 'exotic' sedim ent
(i.e. sedim ent of different grain size), and it is has b een
50
M ar Ecol Prog Ser 253: 3 9 -5 4 , 2003
w ell d ocum ented th at m ortality following deposition
increases w ith sedim ent silt-clay content (e.g. Kranz
1974, M au rer et al. 1981a,b, Turk & Risk 1981, C handrasek ara & Frid 1998, authors' unpubl. data); and
(2) our m ore recen t experim ents have dem onstrated
m uch h ig h er recolonisation of d efau n ated m arine se d ­
im ent additions th an of terrestrial sedim ent additions
(R. W hitlatch & V. C um m ings unpubl. data).
T here w as no consistent distinction b etw een the 3
terrestrial sedim ent treatm ents in term s of their m acro­
faunal com m unities or sedim ent p roperties (with the
exception of protein) during the experim ent. W hile the
protein content of the S70 treatm en t w as significantly
higher th an th at of the S50 treatm en t on all sam pling
dates, this did not translate to differences in th e type or
num ber of m acrofauna colonising the plots over the
tim escale of th e experim ent. T hese results indicate
that th e w ay terrestrial sedim ent enters th e estuary
does not affect the im pact on th e existing m acrofaunal
com m unities or their su b seq u en t recovery. It also veri­
fies an d increases the generality of the results of our
previous sedim entation experim ents, in w hich only
one m ethod of terrestrial sedim ent p rep aration w as
used (Norkko et al. 2002, H ew itt et al. in press, Thrush
et al. in press).
Sediment characteristics correlated with macrofauna
Levels of chi a, P an d N w ere the sedim ent properties
most im portant in structuring the control plot com m u­
nities, and togeth er they ex plained 76% of the v ari­
ability in com m unity com position over the experim ent.
Of these 3 properties, th e am ount of chi a p resen t w as
the m ost im portant. Chi a w as generally 2.5 to 6 tim es
higher in th e control plot sedim ents. The chi a:phaeophytin ratio w as also h ig h er in these plots, indicating
they contained a m uch g reater proportion of available
food (i.e. 5 to 40 tim es more; Fig. 8F) th a n the terrestrial
sedim ent deposits. Interestingly, th e tem poral change
in th e chi a:phaeophytin ratio in th e control plots w as
very similar to that of th e ab u n d an ce an d diversity of
taxa (cf. Figs. 3A,B & 8F), suggesting th at small, re la ­
tively m obile m acrofauna are able to select areas
w hich are 'food-rich' as suitable habitats. Recent
experim ents have also d ocum ented th e positive re la ­
tionship b etw een chi a as a food source an d m acrofau­
nal abundance. For exam ple, Stocks & G rassle (2001)
found recolonisation by m acrofauna w as 62 % low er in
areas w ith red u ced m icroalgal abundance. Similarly,
laboratory experim ents by Nilsson et al. (2000) show ed
that oligochaetes responded negatively to small patches
of food-depleted sedim ents. The d ep leted levels of
chi a in our experim ent m ay be partly d u e to the
extrem e cohesiveness and thickness of th e terrestrial
sedim ent layers preventing m igration of the buried
benthic diatom s back to the sedim ent surface. A study
of the effect of thin silt deposits on m icrobenthos found
that this active m igration of diatom s w as also a key
m echanism for restoring oxygenation of the sedim ent
surface (Wulff et al. 1997). The lack of oxygenation
of the terrestrial sedim ents undoubtedly restricted
the niches available to colonists in our study (e.g. see
Fetzer et al. 2002).
Total sedim ent P w as 3 to 7 tim es higher in the
terrestrial sedim ents th an in the control sedim ents
throughout the experim ent (Fig. 7). Due to the highly
ch arg ed n atu re of both the P and clay particles, it is
likely that the P associated w ith the terrestrial sedi­
m ent treatm ents is very tightly bound and thus not
'available' to the m icrophytobenthos. Sedim ent P,
along w ith % coarse sand, total carbohydrate and p e n ­
etrability, w as also im portant in driving the distinction
betw een the m acrofaunal com m unities of the te rre s­
trial sedim ent and control treatm ents. T ogether these
4 variables explained almost 30% of the variability
in com m unity com position over the experim ent. While
coarse sand w as identified as im portant, th ere w as
never any in the terrestrial sedim ents, and it is likely to
be a surrogate for some other lim iting sedim ent c h ar­
acteristics (Gray 1974). The im portance of p en etrab il­
ity is m ore likely to be a reflection of the 'fluidity' of the
terrestrial sedim ents on the first few sam pling occa­
sions: as these plots dried out, their penetrability
becam e similar to that of the control sedim ents (Fig. 7).
Total carbohydrate content w as considerably elevated
in the terrestrial sedim ent treatm ents. However, a
drop in total carbohydrate content associated w ith an
increase in m icrobial biom ass w as noted on Day 93 in
all terrestrial sedim ent treatm ents (Fig. 8), perhaps
suggesting a slight recovery.
T here w as a m uch higher am ount of organic m atter
associated w ith the terrestrial sedim ents, yet the lack
of colonisation in these plots indicates it w as obviously
not available to the m acrofauna. Stable isotope analy­
sis w as conducted on sedim ent sam ples collected from
each replicate experim ental plot on Day 23 only (using
m ethods described in Gibbs et al. 2001). This revealed
S13C values for the terrestrial sedim ent plots of less
th an -2 5 .9 % for all but 2 replicates (-21.6 and -22.4 %)
com pared w ith values of -13.6 to -14.1 % for the con­
trol plot sedim ents. T hese values indicate that the con­
trol plot sedim ents are m ore enriched w ith 'biologi­
cally available' m aterial th an the terrestrial sedim ents.
O ur study provides further evidence to show that,
w hile terrestrially derived organic m aterial commonly
enters estuaries in relatively large am ounts w hen
associated w ith sedim entation events, it does not n ec­
essarily rep resen t a nutritionally im portant or utilisable
food source (e.g. Sobczak et al. 2002).
Cum m ings et al.: Terrestrial sedim ent deposits in m arine soft-sedim ent habitats
Burrowing crabs
The im portance of bioturbating fauna in increasing
sedim ent resuspension, erodability and nutrient e x ­
chan g e is w ell know n (e.g. Pelegri & B lackburn 1994,
H ansen & K ristensen 1997, M ortim er et al. 1999). A
large influx of juvenile crabs w as indicated by an
ab u n d an ce of small b urrow holes in th e terrestrial se d ­
im ent deposits early on in our experim ent, and there
w as clear evidence of persistence an d grow th of the
crabs (increase in burrow size) on su b seq u ent sam ­
pling occasions. However, our core sam pling did not
detect large num bers of burrow ing crabs in th e plots,
probably d u e to the small size of the corer used. Crab
burrow s w ere not found at all in the control plot sed i­
m ents. A study exam ining th e trophic relationships
b etw een m acrofauna and terrestrial sedim ent deposits
found strong evidence that crabs w hich quickly co­
lonise terrestrial sedim ent deposits are not in fact
feeding on the terrestrial m aterial (Gibbs et al. 2001).
Thus, the rapid colonisation of th e deposits by crabs in
our experim ent (as also noted by N orkko et al. 2002,
H ew itt et al. in press, T hrush et al. in press), is simply a
response to finding a substrate that is m ore suitable for
building an d m aintaining burrow s in rath er th a n the
surrounding sandier sedim ents, and also rath er than
exploiting a n ew food source. In the longer term , the
burrow ing activities of these crabs will be im portant in
the rem obilisation of th e experim ental plot sedim ents,
and th e recovery of th e m acrofauna (Norkko et al.
2002, H ew itt et al. in press, T hrush et al. in press).
Settlement cues
W aterborne, organism - an d process-specific (sensu
W oodin et al. 1995) chem ical cues have b een show n
to induce settlem ent an d m etam orphosis in larvae
of m any m arine in v erteb rates (e.g. Scheltem a 1974,
Pearce & Scheibling 1990, O 'C onnor & Ju d g e 1997,
W oodin et al. 1997). Larvae an d juveniles of some sp e ­
cies can differentiate b etw een d isturbed an d u n d is­
tu rb ed surface (top few mm) sedim ents (Woodin et al.
1995), in response to short-term (h) ch anges in g ra d i­
ents of solutes, such as oxygen (Marinelli & Woodin
2002) and am m onium (Woodin et al. 1998). In the latter
case, arenicolid polychaete recruits rejected sedim ents
w h ere am m onium levels w ere high an d similar to
those found in distu rb ed sedim ents. However, in our
long-term study, p orew ater NH4-N does not ap p ear to
be an im portant cue for colonising m acrofauna given
that concentrations w ere considerably low er in the
terrestrial sedim ent deposits th a n in the control sed i­
m ents throughout th e experim ent. M easurem ents of
other p ore-w ater associated substances m ay be useful
51
in future studies of recolonisation of terrestrial sedi­
m ent deposits. M ixing of porew ater solutes w ith the
overlying w ater w hen the sandflat is im m ersed by the
tide m ay provide an im portant cue for juveniles of
some soft-sedim ent m acrofaunal species th at drift over
the site in the w ater column. T hese juveniles m ay be
able to detect process-specific solute gradients close to
the sedim ent-w ater interface, thereby assessing the
biogeochem ical properties of the surficial sedim ents
(and thus habitat suitability) w ithout having to physi­
cally contact the sedim ent surface. For exam ple, Krug
& Zim m er (2000) show ed that com plex carbohydrates
contained in the porew ater of algal patches that w ere
released into the overlying w ater initiated a b eh av ­
ioural response in opistobranch larvae. Furtherm ore,
they show ed that the am ount released w as greatest
early in the flood tide, suggesting th at larvae carried
into the algal patches during the first few m inutes of a
high tide should have a g reate r chance of settling in
response to this cue th an larvae transported later in the
high tide. This has w ider im plications for the observed
differences in the w ater-colum n associated dispersal of
post-settlem ent stages of soft-sedim ent m acrofauna
d ep en d e n t on the stage of the tidal cycle (e.g. Commito
et al. 1995, C um m ings et al. 1995, W hitlatch & O sm an
1998), and suggests that perh ap s these fauna are
ad ap ted to optim um cue-detection times. Investiga­
tions into these hypotheses are currently in progress.
CONCLUSIONS
This study has provided further evidence of the p e r­
sistence of terrestrial sedim ent deposits on an intertidal
sandflat. A large num ber of sedim ent properties, w hich
are potentially im portant as cues for habitat suitability to
colonising m acrofauna, are rem arkably different to those
of the surrounding sandflat, and show ed little signs of
change in the several m onths following deposition.
D espite this, w e w ere able to identify that m acrofaunal
com m unity com position is strongly correlated w ith the
am ounts of chi a, carbohydrate, P, N and coarse sand in
the sedim ents, levels of w hich w ere considerably d e ­
pleted (in the case of chi a and coarse sand) or elevated
(P, N and carbohydrate) in the terrestrial sedim ents. We
recom m end that future studies of m acrofaunal recoloni­
sation following deposition of terrestrial sedim ent in ­
clude m easurem ent of these sedim ent properties. In a d ­
dition, analyses w hich isolate the biologically available
portions of the carbohydrate, P and N m ay provide even
m ore insight as to the im portance of these properties as
settlem ent cues and indicators of recovery. Finally, the
w ay terrestrial sedim ent enters the estuary does not
ap p ear to effect either its im pact on the existing m acro­
faunal com m unities or their subsequent recovery.
52
M ar Ecol Prog Ser 253: 3 9 -5 4 , 2003
E levated rates of sedim entation in m arine environ­
m ents as a result of h um an activities is a recognised
problem . W ith predictions of h igher rainfall associated
w ith global w arm ing, th ere is a risk that sedim entation
will increase in th e future. Thus, it is im portant to
develop a m echanistic u n d erstan d in g of the im pact
and su b seq u en t recovery of sedim ent deposits in the
m arine environm ent, to en able us to predict the
m agnitude, extent an d persistence of such events on
m arine habitats and, hopefully, to en ab le us to m itigate
any detrim ental effects.
A cknow ledgem ents. T hanks to G reig Furmell, Jo an n e Ellis,
D iane Schultz, Rick Liefting, Terry H um e an d Iain M acD on­
ald for their invaluable h elp in setting u p an d sam pling this
experim ent (and especially for stan d in g u p to th e arm pits in
w ater for hours w aiting for th e tide to go out!). M ax G ibbs
m ade valuable com m ents on a draft m anuscript. E nvironm ent
W aikato kindly gave their perm ission to conduct th e experi­
m ent in th e W hangapoua Estuary. This w ork w as fu n d ed by
NIW A-NSOF (NRK022) an d th e N ew Z ealand Foundation for
R esearch Science an d Technology (FRST-C01X0024).
Appendix 2. Results of gen eralised linear m odelling for tre a t­
m ent effects on sedim ent properties. W here a significant Time
x T reatm ent (T x Tr) in teraction occurred, th e treatm en t pvalue for each day is also given. A lthough block w as included
in th e m odel, th e results are not given h e re for reasons of
brevity. LOI: loss on ignition
Variable
Factor
% Coarse
sand
Time
Treatment
T x Tr
Time
Treatment
T x Tr
0.2612
<0.0001
0.1296
0.0043
<0.0001
0.0010
Time
Treatment
T x Tr
Time
Treatment
T x Tr
<0.0001
<0.0001
0.8251
<0.0001
<0.0001
0.0718
% Clay
Time
Treatment
T x Tr
<0.0001
<0.0001
0.0072
Penetra­
bility
Time
Treatment
T x Tr
Time
Treatment
T x Tr
<0.0001
<0.0001
0.2130
<0.0001
<0.0001
<0.0001
Carbo­
hydrate
Time
Treatment
T x Tr
<0.0001
<0.0001
0.0017
Protein
Time
Treatment
T x Tr
Time
Treatment
T x Tr
Time
Treatment
T x Tr
Time
Treatment
T x Tr
<0.0001
<0.0001
0.2232
<0.0001
0.8044
0.5672
<0.0001
<0.0001
0.9972
0.0004
0.2992
0.0011
Chi a :Phaeo Time
Treatment
T x Tr
0.2581
<0.0001
0.3022
Porewater
n h 4-n
Time
Treatment
T x Tr
<0.0001
<0.0001
<0.0001
% Medium
sand
% Fine
sand
% Sût
Shear
strength
Appendix 1. Results of generalised linear m odelling for tre a t­
m ent effects on m acrofaunal an d colonising m acrofaunal core
data. W here a significant Time x T reatm ent interaction (T x
Tr) occurred, th e treatm en t p-value for each day is also given.
For the num ber of individuals an d n u m b er of tax a for b o th th e
m acrofauna and colonising m acrofauna core data, a Poisson
distribution w as used. W hen a quasi-likelihood function w as
u sed in the m odel, no p -value is given. A lthough block w as
included in th e m odel, th e results are not given h e re for
reasons of brevity
Variable
Macrofauna
No. of ind.
Factor
p-value
Time
(Day)
p-value
Microbial
biomass
Chi a
Phaeo­
phytin
p-value
Time
Treatment
T x Tr
0.0003
<0.0001
<0.0001
127
6
<0.0001
<0.0001
Time
Treatment
T x Tr
0.0718
<0.0001
0.0651
127
6
0.0013
0.0004
Colonising macrofauna
No. of ind.
Time
Treatment
T x Tr
0.0022
<0.0001
0.4151
P
Time
Treatment
T x Tr
<0.0001
<0.0001
0.0001
No. of taxa
No. of taxa
Time
Treatment
T x Tr
0.1107
<0.0001
0.9829
N
Time
Treatment
T x Tr
<0.0001
<0.0001
0.0003
Aquilaspio
aucklandica
Time
Treatment
T x Tr
0.5095
<0.0001
0.3062
%C
Nereids
Time
Treatment
T x Tr
0.0471
<0.0001
0.8243
Time
Treatment
T x Tr
Time
Treatment
T x Tr
0.0050
<0.0001
0.8005
<0.0001
<0.0001
<0.0001
% Organic
content
(LOI)
Time
(Day)
p-value
36
50
93
127
<0.0001
0.3319
0.0001
0.0241
36
50
93
127
36
50
93
127
0.0206
0.1728
0.0250
0.0050
0.0170
0.1831
0.0170
0.0105
36
50
93
127
36
50
93
127
0.0659
0.0029
0.0195
0.0047
<0.0001
0.0050
0.0005
0.0036
36
50
93
127
0.0988
0.1041
0.0643
0.1352
36
50
93
127
36
50
93
127
36
50
93
127
0.0147
0.5118
0.3882
0.0039
<0.0001
0.0125
<0.0001
0.0027
<0.0001
0.0067
<0.0001
0.0141
36
50
93
127
0.0009
<0.0001
0.0014
0.0398
Cum m ings et al.: Terrestrial sedim ent deposits in m arine soft-sedim ent habitats
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Editorial responsibility: John G ray (C ontributing Editor),
Oslo, N orw ay
Subm itted: Ju ly 25, 2002: A ccepted: January 23, 2003
Proofs received from author(s): A pril 29, 2003