Environmental impacts o f offshore wind farm s in the Belgian part o f
the North Sea: Learning from the past to op tim ize fu tu re m o n ito rin g
program m es
Degraer Steven
Operational D irectorate Natural Environm ent, Royal Belgian Institute o f Natural Sciences, Gulledelle
100, 1200 Brussel
The Belgian offshore wind farm m onitoring programm e
Belgium has allocated a 2 38 km 2 zone in the Belgian part o f the North Sea (BPNS) to offshore
renewable energy production, fo r exam ple offshore wind farm s. The firs t wind turbines were b u ilt in
2008. A t present (O ctober 2013), 109 tu rb ine s are operational in the BPNS. The installed wind
turbines d iffe r in fo un da tio n type and generated power: while the firs t six w ind tu rb in e s have
gra vity based fo un da tio ns (GBF), the m a jo rity are m onopiles (55) follow ed by ja c k e t fo un da tio ns
(48). The power th a t can be generated ranges between 3 and 6.1 5 m egawatt (MW) per w ind turbine.
In the next few years, several hundreds o f turbines w ill be up and running. The offshore w ind farm s
are expected to co ntribu te fo r about 43% o f the Belgian 2020 targets fo r renewable energy.
Prior to construction, a developer needs to obtain a dom ain concession and an environm ental
perm it. The latter includes a num ber o f term s and co nd itio ns to m inim ise or m itigate the
environm ental im pact o f the wind farm project. This imposes a m on itoring program m e to assess
the potential impacts on the marine environm ent. These assessments enable the auth orities to
impose m itig a tio n measures or even halt the activities in case o f extrem e damage to the marine
ecosystem. The m on itoring program m e equally allows understanding and evaluating the underlying
ecological processes in su pp ort o f an e nviro n m e nt-frien dly offshore wind farm policy and
management. The program m e started in 2005 and targets physical (hydro-geom orphology and
underw ater noise), biological (epifouling co m m un ity on the hard substratum , macro and epibenthos
o f the so ft substratum , fish, seabirds and marine m ammals), as well as socio-econom ic (seascape
perception and offshore renewables appreciation) aspects o f the marine environm ent.
The Management Unit o f the North Sea M athem atical Models (MUMM), a Scientific Service o f the
Operational D irectorate Natural Environm ent (OD Nature) o f the Royal Belgian Institute o f Natural
Sciences (RBINS) coordinates the m on itoring program m e. To cover all necessary scientific expertise
MUMM collaborates w ith several institutes: the Research Institute fo r Nature and Forest (INBO), the
Institute fo r A g ricu ltu ra l and Fisheries Research (ILVO-Bio-Environmental research group), Ghent
University (Marine Biology Research Group and INTEC), International Marine and Dredging
C onsultants (IMDC) and G rontm ij Belgium NV.
This rep ort presents an integrated overview o f all scientific fin d in g s o f the Belgian offshore wind
farm m on itoring program m e, w ith the specific aim o f drawing lessons from these fin d in g s to
optim ise fu tu re m o n itoring program m es. A series o f anticipated negative and positive im pacts are
covered, but the report also targets an insight in the underlying ecological processes. The report
fu rth e r elaborates on co nte xt setting and nuancing the results, and ends w ith some reflections to
optim ise the fu tu re m on itoring program m e.
A nticipated negative impacts
A nticipated negative impacts on the marine environm ent also affect social acceptance o f offshore
wind farm developm ents. The lack o f social acceptance is actually considered one o f the most
im p o rta n t challenges o f energy project developers w orldw ide. The social acceptance o f offshore
w ind farm s in Belgian waters was investigated th ro u g h questionnaires in 2002 and 2009, i.e. p rior
to and a fte r the firs t wind tu rb ine s had been constructed in 2008. The research dem onstrated an
increasing positive a ttitu de tow ards offshore w ind farm s w ith 68% in su pp ort o f the initiatives in
2009 versus 53% in 2002, and only 8% opponents in 2009 versus 21% in 2002. More than 90% o f
the 2009 respondents considered w ind energy to be a good alternative to non-renewable energy
sources. In Belgium, offshore w ind farm siting is socially and e nvironm entally more acceptable than
onshore w ind farm s, even when seascape is taken into account. Interestingly, g ettin g inform ed on
environm ental impacts o f offshore w ind farm s was valued highest by the public. A fo llo w up study
on social acceptance is proposed when the wind farm s closest to the coast are constructed.
A nticipated negative ecological impacts cover the risks o f increased tu rb id ity , increased sedim ent
erosion and surfacing o f the e le ctricity e x p o rt cable. Detailed m orphological investigations found
th a t increases in tu rb id ity are m ainly due to m eteorological events rather than to the construction
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and operation o f the w ind farm s. Sedim ent erosion stayed w ithin acceptable lim its: the erosion
protection around GBFs w ind tu rb ine s functioned w ith o u t any secondary erosion, while the
m onopile erosion pits ranging fro m 2 to 6.5m were adequately confined by an erosion protection
layer. Flowever, there were substantial sedim ent losses (30 to 35%) during the dredging and
dum ping activities to install the GBFs, leaving a series o f dredging pits th a t have been refilled by
using sand from the second phase o f wind farm developm ent. Electricity e x p o rt cables fu rth e r
proved susceptible to exposure because o f the dynam ic sand dune m igration. A continued
m on itoring o f tu rb id ity using satellite images is advised. The fo llo w -u p w ith m ultibeam o f erosion
near the foundations, w ind tu rb ine sta b ility and cable burial, should be continued.
Negative impacts on seabirds th ro ug h habitat change, habitat loss, barrier-effects and collision are
m ajor environm ental concerns. While some species avoided the wind farm s (i.e. northern gannet
M orus bassanus, com m on g u ille m o t U ria aalge and razo rb ill Alca to rd a at the m ost offshore Bligh
Bank, and com m on guii Larus canus at the m ost onshore T horntonbank), o th e r species seemed to
be attracted (i.e. lesser black-backed guii Larus fuscus and herring guii Larus a rg en ta tus at the
Bligh Bank, and little guii Hydrocoloeus m inutus, great black-backed guii Larus m arinus, Sandwich
tern Sterna sandvicensis and com m on tern Sterna hirun d o at the T horntonbank). Large gulls were
often seen flyin g at ro to r height (15-22%). Based on daytim e observations, each year up to about
1300 birds, m ainly gulls, are expected to collide w ith the tu rb ine s once all w ind farm s w ill be
operational in the BPNS. D uring strong m igration periods, th ru sh Turdus spp. collisions can reach
200 victim s during a single night. Visual census com bined w ith radar observations w ill aid a fu tu re
accurate bird m o rta lity assessment. Future m on itoring o f the local seabird d is trib u tio n w ill fu rth e r
increase the likelihood o f displacem ent effect detection and w ill allow discerning possible
habituation effects.
Increased noise levels generated by w ind farm s, may harm the marine environm ent. For example,
the m axim um detected above w ater sound pressure level during pin piling activities fo r the
installation o f ja cke t foundations, reached 145 dB(A). The operational sound pressure level m ainly
generated by the blades passing th ro ug h the air, am ounted to 105-115 dB(A) at w ind speeds higher
than 12m /s and could hence be detected up to a distance o f 10km. U nderwater noise generated
during the installation o f gra vity based fo un da tio ns (about 115 dB re 1 pPa root mean square) was
close to am bient noise levels. In contrast, m onopile piling produced excessive underw ater noise
levels o f 179-194 dB re IpPa (zero to peak level at 750m), attenuating to am bient noise levels at a
distance o f up to 70km . For pin piling (jacket foundations) lower noise levels o f 1 72-1 89 dB re 1 pPa
were measured, but the to ta l num ber o f blows per m egawatt installed is 57% higher than fo r a
m onopile. When in operation, steel m onopile sound pressure is double o fth a t em itted by a jacke t
fo un da tio n tu rb ine , in its tu rn twice the sound pressure o f the background or GBF foundation
turbine. Future m on itoring w ill m ainly ta rg e t continuous underw ater noise m easurem ents, which
can be com pared w ith o the r types o f hum an-induced noise in the marine environm ent.
Piling noise in fact is a m ajor concern to marine mammals and fish. For the harbour porpoise
Phocoena phocoena occurring in Belgian waters w ith densities o f up to 2.7 in d /k m 2, aerial surveys
during a piling event showed a distance o f disturbance o f porpoises o f up to at least 20km fro m the
piling location. A m odel allowed reproducing the porpoise displacem ent in a wide area around the
piling zone, but outside th is area larger differences between the observations and the m odel were
detected. The latter d ifficu ltie s may be caused by the spatial va ria b ility in food availab ility or
seasonal movements. Further fine tu n in g and testing o f the m odel in d iffe re n t piling conditions and
based on aerial surveys and passive acoustic m on itoring data, is therefore advised. The im pact o f
construction and operational noise on fish eggs and larval developm ent in Belgian waters only
started recently, and needs more atte ntio n in the fu tu re m on itoring program m e. These ‘ passive
d rifte rs ’ cannot actively escape from the exposure to hum an-induced noise. Especially fish w ith a
swim bladder, fo r which the European sea bass D icentrarchus la b ra x w ill be used as a model
species, w ill be targeted th ro ug h an e xperim ental study.
A nticipated positive impacts
The protection against fish in g activities inside the w ind farm s is u ndoubtedly the main anticipated
positive im pact o f offshore wind farm s on the marine environm ent. Based on VMS data (Vessel
M onitoring System), it can be concluded th a t fish in g vessels - m ainly traw lers - are v irtu a lly
everywhere in Belgian waters, except in the w ind farm s. A m oderate increase in fish in g activities,
m ostly fro m Dutch fish in g vessels, is noted in the zone surrounding the w ind farm concessions.
Recreational anglers, m ostly ta rg etin g pelagic and benthopelagic fish, firs t concentrated close to
the gravity-based fo un da tio ns at the T horntonbank, but recently seem to have alm ost left the area.
Com bined w ith a possible reef effect, the exclusion o f fisheries was expected to have a sign ifican t
positive im pact on the so ft sedim ent benthos. A m acrobenthic Nephtys c irro sa co m m un ity was
found at the T ho rnto nb a nk and Gootebank, corresponding to a typical sedim entology w ith a median
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grain size between 331 pm and 410pm . The co m m un ity is dom inated by a few species, like the
polychaetes N. cirro sa and Spiophanes bombyx, the mysid shrim p Gastrosaccus s p in ife r and the
am phipod Urothoe brevicornis. A natural interannual v a ria b ility in densities between about 200 and
800 in d /m 2, was detected. While the m acrobenthic co m m un ity structure was sim ilar in the control
and concession sites before construction, sign ifican t differences were found in 2008. In th is year,
i.e. sh o rtly a fte r m ajor construction w orks, high densities (dom inated by S. bombyx) were detected.
However, no large scale effects could be detected, as the differences between co ntrol and im pact
sites disappeared again after tw o years. The Benthic Ecosystem Q uality Index (BEQI) confirm ed the
recovery o f the co m m un ity a fte r the construction works. Long term and larger-scale effects could
hence not be detected. Future m on itoring w ill focus on the fisheries exclusion and smaller-scale
enrichm ent effects, and aim at detecting these effects at the scale o f a com plete w ind farm .
Demersal fish, benthopelagic fish and epibenthos from so ft sedim ents may be p ositively impacted
as well. For exam ple, epibenthos biomass and length o f w hitin g M erlangius m erlangus slig h tly
increased at the edge o f the T horntonbank, while (tem porarily) increased abundances o f sole Solea
solea and dab Lim anda lim a n d a were observed at the edge o f the Bligh Bank. Inside the w ind farm s,
several local and tem poral impacts were detected. A t the Thorntonbank, increases in dab mean
length (2012), epibenthos biomass (2009) and num ber o f demersal fish species (2009) were
observed. A t the Bligh Bank, densities o f the com m on starfish A sterias rubens and sole increased
over the m on itoring period, and several ‘ large r’ plaice Pleuronectes platessa and tu rb o t Psetta
m axim a were noted. Some short tim e construction effects were seen sh ortly after the start o f the
p iling activities, like increased sandeel A m m odytes tobianus densities and decreased densities in
dab, o ph iu roids O phiura ophiura, squid A llo th e u tis su bu la ta and dragonet C allionym us lyra. A
continued m on itoring , ta king into account the high natural spatio-tem poral variability, w ill ensure
an increased power o f im pact detectability.
Some fish may be d ire ctly attracted to the a rtificia l hard substrata, in search fo r food or shelter. The
offshore wind tu rb in e fish co m m un ity (near the gravity-based foundations) was dom inated by
pouting Trisopterus luscus and cod Gadus m orhua, while also o the r species such as poor cod
Trisopterus m inutus, saithe Pollachius virens and black seabream Spondyliosom a cantharus were
exclusively detected close to the turbines. Cod and pouting catches were up to 12 and 30 tim es
higher, respectively, compared to the wrecks, and up to > 100 tim es higher com pared to the nearby
sandy areas. The density peaks o f both species (May-November fo r cod and September-December
fo r pouting), probably reflect a seasonal spawning m igration. Young individuals dom inated the local
cod and pouting populations. Future m on itoring w ill focus on the representativeness o f GBF wind
turbines com pared to steel m onopile turbines, the latter having a sm aller erosion protection layer
and are positioned in more offshore waters.
Fouling organism s colonising a rtificial hard substrata, increase the local species richness. D ifferent
com m unities can be detected along the depth gradient: the marine splash midge Telm atogeton
ja p o n icu s dom inated the splash zone; the intertida l fring e was characterised by barnacles and the
blue mussel M ytilus edulis', a Jassa-Tubularla-Actiniana co m m un ity in the subtidal zone was
dom inated by the am phipod Jassa h erdm an i (up to 3 105 in d ./m 2) and the hydroids Tubularia
indivisa and T. la ry n x (up to 90% coverage). The patterns in species richness, d en sity and coverage
were best illustrated at the T horntonbank, where th ey showed an increase m ainly during the firs t
tw o to three years, after which th ey stabilised. These long-term dynamics are superim posed by
seasonal dynamics w ith highest densities (generally ranging between 1-1.5 105 in d ./m 2) and
coverage (on average 60-70%) in spring and summer. In addition to the settling o f new species,
com p etitio n and predation are im p o rta n t biological processes shaping hard substrata com m unities.
Future m on itoring w ill focus on a better understanding o f the spatial heterogeneity, the dynamics
along the onshore-offshore gradient, and the use o f the a rtificia l reefs by larger invertebrates, such
as crabs and lobsters.
Understanding ecological process behind the observed impacts
A proper understanding o f the ecological processes underlying the observed impacts is
indispensable to deliver science-based advice fo r an e nviro n m e nt-frien dly design o f fu tu re wind
farm s. For exam ple, understanding the effects o f organic enrichm ent on so ft sedim ent
m acrobenthos at a small scale, allows e xtra p o la tin g these small-scale effects to large-scale and long
term impacts. Lower median grain sizes o f the sedim ent and increased organic m atter levels were
found close to the gra vity based fo un da tio ns at the Thorntonbank. These phenomena could be
linked to a m acrobenthic co m m un ity evolving away fro m the typical N. c irro s a co m m un ity in this
area. Close to the turbines, elevated m acrofaunal densities (up to 11 500 in d ./m 2), biomasses (up to
9540 m g /m 2) and num ber o f species (up to 32 spp.) were found, especially along the Northwest
and Southwest transects. Juvenile com m on starfish A. rubens, the sand mason Lanice conchilega,
the bee spionid S. bombyx, and the typical hard substrate species M onocorophium acherusicum and
J. h erdm an i tend to dom inate in th is enriched environm ent. The local enrichm ent was detectable to
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a distance o f 50m from the turbines. Future m on itoring w ill ta rg et the spatial extension o f this
effect th ro ug h tim e and at o the r types o f wind tu rb ine foundations.
Zoom ing into fish habitat use in Belgian offshore w ind farm s, m ainly young individuals o f A tla n tic
cod and pouting were clearly attracted nearby the wind turbines as was observed by divers and by
line fish in g. Demersal fish species were however not found to be consistently attracted at larger
distances (m inim um 1 80m ) from the turbines. So far, no clues o f increased recruitm ent or grow th in
demersal species were detected at larger distance. Flowever a num ber o f larger individuals o f plaice
Pleuronectes platessa were caught at the Bligh Bank. Dab on the o the r hand occurred in lower
num bers, but rem arkably had a fu lle r stomach inside (mean Fullness Index, FI: 0.15) than outside
(FI: 0.05) the area. Sim ilarly to cod, p outing showed feeding m ainly upon e pifo u lin g species, such as
J. h erdm an i and Pisidia longicornis. Cod indeed showed an attraction to the a rtificia l hard substrata
w ith about 90% o f the individuals staying w ith in a 40m range from the w ind turbines. Future
m on itoring w ill focus on a ttraction and p roduction mechanisms o the r than food availability, but w ill
also aim at including a w ider set o f fish species and an energy p ro filin g o f th e ir prey species.
Recent sightings o f European shag Phalacrocorax aristotelis, a seabird species favouring cliffs and
rocky shores, in Belgian w ind farm s and black-legged kittiw akes Rissa trid a c ty la starting to breed
on North Sea gas platform s, all p o in t tow ards the a ttra ctio n -p ro d u ctio n potential o f offshore wind
farm s fo r seabirds. W hether birds are attracted to wind farm s fro m a sheer physical p o in t o f view,
w ith the w ind farm fu n ctio n in g as a stepping stone or a resting place (attraction), or w hether they
already learned to e x p lo it the possibly increased food availab ility (production), rem ains to be
investigated. Black-legged kittiw akes were already regularly observed foraging inside the Bligh Bank
wind farm , w ith the percentage o f kittiw akes actively fo ra gin g inside the w ind farm being much
higher than in the co ntrol area (5.9% versus 0.3%). Also high num bers o f lesser black-backed gulls
were foraging close to the T ho rnto nb a nk ja c k e t foundations. Future m on itoring w ill pay atte ntio n to
the behaviour and fo ra gin g related activities o f seabirds, and to pelagic fish as the m ost im p orta nt
prey species fo r seabirds.
W ithin the a ttractio n-pro du ction debate o f offshore w ind farm s we also investigated w hether marine
mammals were attracted to the increased fish abundance close to w ind tu rb ine s or rather repulsed
by the increased noise levels. Flarbour porpoises showed an uneven sp atiotem poral d is trib u tio n in
Belgian waters, w ith a s h ift from the northern and north-eastern part o f the Belgian waters tow ards
the south-w est and west between February and A p ril. As the offshore wind farm s are relatively small
compared to the area th a t can be covered in a short tim e period by th is highly m obile species,
differences in d is trib u tio n o f h arbour porpoise w ith in and outside wind farm s are probably inferio r
to seasonal variations w ithin the southern North Sea caused by m ovements to fin d suitable prey
resources. In a dd itio n to continued aerial surveys, fu tu re m on itoring w ill ta rg et small scale passive
acoustic m on itoring (PAM) to investigate the potential use o f offshore w ind farm s by harbour
porpoises. A tte n tio n w ill also be paid to disentangle the com plex link between PAM data and
species densities.
Nuancing impact interpretation
Several impacts have been identified in the Belgian offshore w ind farm s, varying from seem ingly
negative to seem ingly positive impacts. Species richness increased because hard substrata (wind
tu rb ine fo un da tio ns and erosion protection layers) were introduced. Flowever, offshore w ind farm s
may also increase the risk o f invasions in the North Sea, as non-indigenous species (NIS) may now
fin d more suitable place to survive and hence strengthen th e ir com petitive position in the North
Sea. More than h alf o f the hard substratum inte rtid a l species (e.g. the invasive Pacific oyster
Crassostrea gigas) in the w ind farm s can be categorised as NIS. Pouting is attracted to wind farm s,
but we do not know yet w hether these offshore wind farm s act as an ecological trap. Flowever,
pouting is sig n ifica n tly larger inside than outside the w ind farm s, th e ir stomach is fille d more and
th e ir co ndition is sim ilar, so no evidence was obtained to assume th a t the habitat q u a lity o f offshore
wind farm s does not fu lfil the fu n ctio n a l needs o f pouting. Prelim inary e xtra po latio n o f bird
collisions (at North Sea population scale) to fu tu re expansions o f offshore wind farm s showed that
the e xistin g a du lt m o rta lity fo r instance o f lesser and great black-backed guii m ig h t exceed the
accepted threshold o f 5%. Future m on itoring w ill take account o f the need fo r up scaling to species
population levels and the expansion o f offshore w ind farm s in the North Sea.
Increased species richness, densities and biomass fu rth e r can be evaluated in d iffe re n t spatial
settings, fo r instance at tu rb ine level or at the level o f a wind farm or even the Belgian part o f the
North Sea. The species pool o f so ft sedim ent fish and squid did not change drastically, but the
num ber o f hard substrate associated fish species increased from 2 to 8 inside the wind farm
concession area. The num ber o f benthic species in the concession area more than doubled, fro m 91
to 264 species, since the installation o f the firs t tu rb in e foundations, m ainly because o f the increase
o f hard substratum species from 10 to 100. A u tu m n benthic biomass increased ~4000 tim es at the
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scale o f a single gra vity based fo un da tio n from 0.6kg ash-free d ry w eight (AFDW) before
construction to ~2500kg a fte r construction, w ith the m ajor part o f the biomass at the scour
protection (89%) and the intertida l M. edulis zone (10%). For the entire T ho rnto nb a nk w ind farm , the
autum n biomass increased about 14 tim es from about 5 to 70 ton AFDW. The offshore wind farm s
may co ntribu te about 3% o f the to ta l biomass in the BPNS. Future m on itoring w ill focus on a
validation o f the fo u lin g biomass estim ates fo r ja c k e t and m onopile foundations.
Reflections fo r an optim isation o f futu re m onitoring program m es
Six years o f m on itoring triggered a reflection on how to best continue the m o n itoring program m e,
building on both basic and targeted m on itoring contexts. The basic m on itoring should be
rationalised at the level o f the likelihood o f im pact detection, related to research e ffo rt and impact
size. The m eaningfulness o f im pact size deserves o u r atte ntio n and should be aligned w ith the
current im plem entation o f European Directives, such as the Marine Strategy Framework Directive.
Future basic m on itoring fin a lly needs to consider the representativeness o f the current fin ding s, so
fa r largely focused on GBFs. W ithin a targeted m on itoring context, the a rtificia l reef effect w ill
undoubtedly play a key role in the fu tu re m on itoring program m e. It already received a lot o f
atte ntio n in the m on itoring so far, but various cause-effect relationships, m ainly linked to the
a ttractio n-pro du ction hypothesis, remain yet to be tackled, preferably th ro ug h international
scientific collaboration. A m ajor challenge however is to achieve a reliable assessment o f cum ulative
impacts and to upscale locally observed impacts to the larger scale at which ecological processes
take place. This w ill require a close collaboration between scientists, ind ustry stakeholders and
adm inistrators, preferably across countries bordering the North Sea.
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