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Appendix S1
Phytoplankton sampling and community description
Phytoplankton samples processing
Quantitative samples were fixed with Lugol’s solution and counted under an inverted
microscope at 400x magnification using the settling technique of Utermöhl (1958).
Picoplankton (organisms smaller than 2 µm in diameter) cannot be identified using this
technique and thus were not considered. The organism was considered as the unit
(unicell, colony or filament). Individual volume (V, m3) and surface area (S, m2)
were calculated according to Hillebrand et al. (1999), based on geometric shapes and
average dimensions (length, width and depth, in µm) of 5–40 organisms per taxa,
measured in each sample under a transmitted light microscope at 1000x magnification.
Also recorded were cell numbers per colony and organism dimensions, including the
maximum linear dimension (MLD, m). The presence of categorical traits, including
aerotopes, flagella, mucilage, heterocytes and siliceous exoskeletal structures, were
noted for each relevant organism. Density of biomass was expressed as biovolume (m3
L-1) by multiplying sample abundance (org L-1) by species individual volume (m3).
Duplicate samples of 350 ml fixed with Lugol solution were used for quantitative
analyses. Samples were gently homogenized and poured into sedimentation chambers of
1 to 10 ml, depending on the density of organisms per sample. Sedimentation time was
4 to 48 hours according to the height of the chamber (Sournia 1978) to allow the settling
of big and small organisms. Duplicate phytoplankton counting were performed by
transects until obtaining 400 units (cells or colonies) according to Utermöhl (1958),
using a Nikon Diaphot inverted microscope (400 X or 1000X) with phase contrast. With
this approach it was possible to count, without introducing bias, organisms bigger than 3
µm diameter. The software ALGAMICA 4.0 (Gosselain & Hamilton 2000) was used for
counting and calculating abundance per milliliter.
Taxonomic identification was based on fresh, fixed and oxidized material (for diatoms).
Flagellates less than 10 µm diameter were not possible to identify at specific level, and
thus, they were classified in different taxa according to their morphology (size, shape,
type of plast, number of flagella). In few cases, diatom species were confirmed under
scanning electronic microscope (JEOL JSM-5200).
The classification system of Hoeck et al. (1993) was followed for Classes and
Divisions, and Krammer & Lange-Bertalot (1986; 1988; 1991), Tomas (1998) and
Round et al. (1992) were used as main taxonomic references for genera and species.
Morphology-based functional groups (MBFGs)
Species were classified into MBFG according to Kruk et al. (2010), based on their
individual Volume (V), Surface (S), S/V, Maximum linear dimension (MLD) and the
presence or otherwise of five categorical traits named: aerotopes, flagella, mucilage,
heterocysts and siliceous exoskeletal structures. This classification included the
following seven groups: (I) small organisms with high S/V, (II) small flagellated
organisms with siliceous exoskeletal structures, (III) large filaments with aerotopes,
(IV) organisms of medium size lacking specialised traits, (V) flagellates unicells with
medium to large size, (VI) non-flagellated organisms with siliceous exoskeletons and
(VII) large mucilaginous colonies. A detailed description of the MBFGs can be found
elsewhere (Kruk et al. 2010).
Significance of empirical entropy peaks
We determined the significance of entropy (S) peaks by comparing observed entropy
against an expected uniform distribution under the null hypothesis of homogeneous
entropy. One thousand comunities were created by sampling the volumes of species
from a random uniform distribution bounded by observed individual volumes. Then,
each species had a biovolume assigned to it, which was taken from a randomization of
the observed biovolume matrix, keeping both the empirical species rank-abundance
pattern and total biovolume in the sample. Subsequently, entropy was estimated for
each segment of the niche as described in methods. For each segment, the observed S
value was compared with the distributions of S generated under the null hyphothesis,
with significance defined according to standard 5% criterion.
Phytoplankton species
Representative phytoplankton species of morphology-based functional groups (MBFGs)
found in Rocha Lagoon at medium and large size-classes. A detailed list of species can
be found in Bonilla et al. (2005).
Species representatives
Medium (~1000 m3)
Large (<10000 m3)
MBFG V
Eutreptiella gymnastica,
NA
Trachelomonas sp.,
Prorocentrum minimum,
Pyramimonas sp. and
Cryptomonas sp.
MBFG VI
Nitzschia spp., Fragilaria
Melosira moniliformis,
sp. and Cyclotella
Thalassiosira eccentrica,
meneghiniana
Surirella splendida,
Cylindrotheca gracilis and
Nitzschia obtusa
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