Particle retention efficiency by a coastal ecosystem in the northeastern Atlantic Ocean.

Abstract

Based on filtration rates obtained in situ at several habitats (mud flats, sand flats, and seagrass beds of Cymodocea nodosa), particle retention efficiency of benthic communities was estimated at Ria Formosa, a coastal lagoon in southern Portugal. The distinct physical characteristics of the water flow (water depth, current speed) at different tidal regimes (spring tide and neap tide) were also considered. Median filtration rates ranged 5.0−45.0 l m−2 h−1, despite the lack of dense bivalve beds or reefs. Based on median filtration rates of benthic communities on the studied substrate types, a model we developed showed that seagrass bed areas retained 15% of suspended particles during spring tide and 47% during neap tide. These retention efficiencies were much higher than those observed on mud flats (2.8% during spring tide, and 8% during neap tide), sand flats of finer grain size (8.8% during spring tide, and 50% during neap tide), and sand flats of coarser grain size (7% during spring tide, and 31% during neap tide). Removal of particles from water column was higher during the ebb tide than during flow tide, with particles of <5 μm diameter (especially diatoms, flagellates, and small-sized particulate matter) being the most commonly removed.

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123
Wetlands
Official Scholarly Journal of the Society
of Wetland Scientists

ISSN 0277-5212
Volume 31
Number 6

Wetlands (2011) 31:1175-1185
DOI 10.1007/s13157-011-0228-x
Particle Retention Efficiency of a Coastal
Ecosystem in the Northeastern Atlantic
Ocean
Vanda Mariyam Mendonça, Martin
Sprung, Margarida Castro & Adelino
Canário

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123
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ARTICLE
Particle Retention Efficiency of a Coastal Ecosystem
in the Northeastern Atlantic Ocean
Vanda Mariyam Mendonça & Martin Sprung &
Margarida Castro & Adelino Canário
Received: 6 December 2009 /Accepted: 31 August 2011 /Published online: 29 September 2011
# Society of Wetland Scientists 2011
Abstract Based on filtration rates obtained in situ at several
habitats (mud flats, sand flats, and seagrass beds of
Cymodocea nodosa), particle retention efficiency of benthic
communities was estimated at Ria Formosa, a coastal lagoon
in southern Portugal. The distinct physical characteristics of
the water flow (water depth, current speed) at different tidal
regimes (spring tide and neap tide) were also considered.
Median filtration rates ranged 5.0−45.0 lm−2h−1, despite the
lack of dense bivalve beds or reefs. Based on median
filtration rates of benthic communities on the studied
substrate types, a model we developed showed that seagrass
bed areas retained 15% of suspended particles during spring
tide and 47% during neap tide. These retention efficiencies
were much higher than those observed on mud flats (2.8%
during spring tide, and 8% during neap tide), sand flats of
finer grain size (8.8% during spring tide, and 50% during
neap tide), and sand flats of coarser grain size (7% during
spring tide, and 31% during neap tide). Removal of particles
from water column was higher during the ebb tide than
during flow tide, with particles of <5 μm diameter
(especially diatoms, flagellates, and small-sized particulate
matter) being the most commonly removed.
Keywords Benthos-plankton interactions.Filter-feeding
impacts.Modelling.Ria Formosa lagoon
Introduction
Sheltered coastal areas are rich in nutrients and
phytoplankton, and the relatively strong currents induce
suspension of particles, favoring the formation of dense
colonies of benthic suspension feeders. Mussel-beds and
oyster-reefs form particularly important communities of
filter-feeders (Dame 1993), but ciliates and bryozoans
(Epstein et al. 1992), cnidarians (Mendonça et al. 2010),
polychaetes (Riisgård 1991), and prosobranch gastropods
(Brendelberger and Jürgens 1993) may also play an
important role in removing particles from seawater, limit
phytoplankton biomass in the overlying water column
(Riisgård 1988; Grant and Bacher 2001); Grant et al.
2005, and control pollution (STAC 2002).
Individual filtration capability and efficiency, and food
size and quality may determine the extent of the impacts of
filter-feeders (Sprung and Rose 1988). Filtration rates and
their impacts are difficult to quantify (Riisgård 2001;
Bartell 2002), thus modelling may be necessary to consider
the numerous variables, including the physical character-
istics of the overlying flow. Models based on bioenergetics
(Madenjian 1995) and hydrodynamics (MacIsaac et al.
1992, 1999; Duarte et al. 2003; Edwards et al. 2005) have
been developed, but have been applied only to monospe-
cific bivalve beds or reefs.
Our study used a model based on both filtration rates of
benthic communities (including non-bivalves) and specific
characteristics of the overlying water flow to quantify
particle retention efficiency of a coastal lagoon in south-
western Europe.
V. M. Mendonça (*):M. Sprung:M. Castro:A. Canário
Algarve Marine Sciences Centre (CCMAR),
University of Algarve,
Gambelas,
8005-139 Faro, Portugal
e-mail: drvandamendonca@mail.seaturtle.org
Wetlands (2011) 31:1175–1185
DOI 10.1007/s13157-011-0228-x
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Methods
Study Site
The Ria Formosa coastal lagoon, southern Portugal,
extends for 55 km bordering a seaward (3-km wide) belt
of dunes interrupted by seven bars, comprising two
peninsulas and six islands. The lagoon and adjacent
terrestrial habitats form the Ria Formosa Natural Park, a
protected wetland of global importance under the Ramsar
Convention 1971.
About 50% of the 90×106m3water volume in neap high
tides, and 75% of the 140×106m3in spring high tides is
exchanged with the Atlantic Ocean (Águas 1986). On the
Spain
Portugal
ATLANTIC OCEAN
NORTH SEA
ANCÃO
PENINSULA
1 Km
Sublittoral and
Oceanic Environments
Intertidal Environments Supralittoral and
Terrestrial Environments
SITE 2
Coarse Sand Flat
SITE 4
Mud Flat
SITE 3
Seagrass Bed ofZostera noltii
SITE 1
Fine Sand Flat
Fig. 1 Sampled sites at Ria Formosa lagoon, southwestEurope, for benthic communities composition and filtration rates
Buoy
Manual
mixer
Dark plastic cover
held by elastic band
Tube for algae
injection
and sample
collection
PVC corer
(OPEN bottom)
MEASURING FILTRATION RATES OF
ALL COMMUNITIES IN THE CORER
a
MEASURING FILTRATION RATES OF
PLANKTONIC COMMUNITIES
(CONTROL FOR BENTHIC COMMUNITIES)
b
Sediment
PVC corer
(CLOSED bottom)
Seawater surface
Fig. 2 Diagram of the apparatus used to measure in situ filtration
rates of benthic communities. Each experiment consisted of a set of
two corers: one to measure filtration rates of all organisms, benthic
and planktonic (a), and one to measure filtration rates of planktonic
organisms (b). Filtration rates of benthic communities were obtained
by deducting the filtration rate of planktonic organisms
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Table 1 Benthic invertebrates (>5μm) identified in the present study at the sampled sites at Ria Formosa lagoon, southern Portugal, and
respective density and biomass (AFDM=ash free dry mass)
ClassSpeciesPredominant feeding strategyAbundance (ind. m−2)Standing biomass (g AFDM m−2)
ANTHOZOANSunidentified (polyp)
Anemonia sp.
Drepanophorus sp.
Sipunculus nudus
Aricia latreillei
Clymene oerstedii
Glycera convoluta
Heteromastus filiformis
Mellina palmata
Hyalonoecia fauveli
Nephthys hombergii
Notomastus sp.
Nereis diversicolor
Owenia fusiformis
Terebella lapidaria
Aspeudopes latreillei
Isopoda (unidentified)
Ampelisca brevicornis
Pagurus sp.
Upogebia pusilla
Carcinus maenas
Diogenes pugilator
Ethusa mascarone
Bittium reticulatum
Caliptraea sinensis
Cerithium vulgatum
Cyclope neritea
Gibbula umbilicalis
Jujubinus depictus
Mesalia brevialis
Nassa incrassata
Nassa reticulata
Nassa sp.
Lunatia catena
Rissoa sp.
Natica sp.
Abra ovata
Cerastoderma edule
Donax trunculus
Dosinia exoleta
Eastonia rugosa
Gastrana fragilis
Loripes lacteus
Nucula nucleus
Psammophila magna
Scrobicularia plana
Solen marginatus
Spisula solida
Suspension-feeder
Suspension-feeder
Carnivore
Deposit-feeder
Deposit-feeder
Deosit-feeder
Carnivore
Deposit-feeder
Suspension-feeder
Deposit-feeder
Carnivore
Deposit-feeder
All strategies
Suspension-feeder
Deposit-feeder
Deposit-feeder
Deposit-feeder
Deposit-feeder
Deposit-feeder
Suspension-feeder
Deposit-feeder
Carnivore
Deposit-feeder
Herbivore
Herbivore
Herbivore
Herbivore
Herbivore
Herbivore
Suspension-feeder
Herbivore
Herbivore
Herbivore
Carnivore
Herbivore
Herbivore
Suspension-feeder
Suspension-feeder
Suspension-feeder
Suspension-feeder
Suspension-feeder
Suspension-feeder
Suspension-feeder
Suspension-feeder
Suspension-feeder
Suspension-feeder
Suspension-feeder
Suspension-feeder
0–1.5
0−1.5
0–1.5
0–3.0
0–1.5
0–15.4
0–15.4
0–20.0
0–1.5
0–9.2
0–33.9
0–21.6
0–15.4
0–72.3
0–1.5
0–307.7
0–3.1
0–15.4
0–3.1
0–30.8
0–3.1
0–27.7
0–3.1
0–353.9
0–3.1
0–26.0
0–15.4
0–1.5
0–612.9
0–1015.4
0–3.1
0–21.5
0–1.5
0–1.5
0–1.5
0–1.5
0–12.2
1.5–13.8
0–1.5
0–4.6
0–1.5
0–1.5
3.1–58.5
0–7.7
0–1.5
0–4.6
0–7.6
0–3.0
0–0.5
0–0.5
0–1.5×10−3
0–30.4
0–4.6×10−5
0–4.6×10−4
0–4.6×10−4
0–0.1
0–4.6×10−5
0–2.8×10−3
0.14
0–6.5×10−3
0–0.1
0–0.1
0–1.5×10−3
0–0.2
0–9.2×10−4
0–0.1
0–0.1
0–0.1
0–9.2×10−3
0–0.1
0–0.1
0–0.4
0–9.2×10−3
0–0.1
0–0.1
0–1.5×10−4
0–0.2
0–1.0
0–6.1×10−3
0–0.1
0–1.5×10−3
0–1.5×10−3
0–1.5×10−3
0–1.5×10−3
0–4.3
0.7–10.3
0–14.1
0–39.0
0–1.8
0–7.7
0.2–2.9
0–0.4
0–15.6
0–28.5
0–39.0
0–10.7
NEMERTEANS
SIPUNCULIDS
POLYCAHETES
CRUSTACEANS
GASTROPODS
BIVALVES
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eastern side of the lagoon, the Gilão River introduces
freshwater which reduces local salinity. On the western
side, salinity ranges from 35.5 to 36.9 ppt year-round
(Falcão et al. 1985), although it can be reduced for short
periods after heavy rains. Tidal elevation inside the lagoon
is 1.3 and 2.8 m, respectively, at mean neap tide and mean
spring tide. Maximum and minimum areas covered by
water during spring tides are 63.1 and 14.1 km2, respec-
tively (Águas 1986).
Within the lagoon, mean concentration of chlorophyll a
ranges 2–3 mg Chl-a m−3(Newton et al. 2003), and
planktonic net primary production has been estimated as
45 gC m−2yr−1(Sprung 1994). Seagrass beds are relatively
abundant (up to 1752 shoots m−2, corresponding to 945 g
Table 1 (continued)
Class SpeciesPredominant feeding strategyAbundance (ind. m−2) Standing biomass (g AFDM m−2)
Venerupis aureus
Venerupis decussata
Leptosynapta inhaerens
Phallusia mammillata
Branchiostoma lanceolatum
Suspension-feeder
Suspension-feeder
Deposit-feeder
Suspension-feeder
Suspension-feeder
0–6.1
0–3.0
0–1.5
0–1.5
0–3.1
0–18.9
0–9.5
0–0.1
0–0.8
0–0.3
ECHINODERMATA
CHORDATES
FINE SAND FLAT
SEAGRASS BED
COARSE SAND FLAT
MUD FLAT
1800 150012009006003000
Euclidean Distance
a
b
c
Fig. 3 Diversity indices by
habitat type of benthic commu-
nities at Ria Formosa lagoon (a),
and comparisons between
benthic communities using
filter-feeders biomass CA case
scores (b), and cluster analyses
(Euclidean distance) based on
density (ind. m−2) and biomass
(g AFDM m−2) of benthic
organisms (c). CA is
Correspondence Analyses,
and AFDM is ash free dry mass
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dry mass m−2) with Zostera noltii on substrates rich in silt
and clay and Cymodocea nodosa on sandier substrates
(Alberto et al. 2001; Cunha and Duarte 2007). Intertidal flats
in the lagoon have secondary production ranging 22.4–54.7 g
ash free dry mass m−2yr−1(Sprung 1994).
Filtration Rates of Benthic Communities in Different
Habitat Types
Filtration rates (FR) are typically described as the rate of
removal of particles from the water-column, over time
(Mann 1976), FR=dlnC/dt, where C is the concentration of
suspended particles, and t is time. Therefore, for a whole
benthic community occupying a certain area of the seabed
(A), the overall filtration rate of a given volume of water
(V) is defined as follows: FR(l m−2h−1)=[dlnC/dt]xV/A.
Using this principle, we assessed benthic communities at
Ria Formosa lagoon in four habitat types: fine sand flats
(median particle size was 0.3 mm, n=5), coarse sand flats
(median particle size was 0.7 mm, n=5), mud flats (median
particle size was 0.008 mm, n=5), and seagrass beds of
Cymodocea nodosa (median particle size was 0.063 mm,
n=5) (Fig. 1). On each habitat type, we enclosed 17 l
seawater, enriched by a macroalgae suspension, in PVC
dark corers (n=10 sets of two corers as shown in Fig. 2,
except for mudflat n=5). Contents were gently mixed
manually, and after allowing 20 min for the system to
adapt, we collected seawater samples (100 ml each, n=3,
every 20 min for 2 h) from inside corers, to estimate
filtration rates. Experiments were conducted at 1-m depth,
during the ebb tide from April to October 1991.
A suspension of microalgae of known diameter (spher-
ical and non-colonial) was added to the system to be more
easily detected by a calibrated Coulter counter (Sheldon
and Parsons 1967). The microalgae solutions of Dunalliella
sp. were cultivated in a medium with pH stabilizer (50 g per
200 ml of distilled H2O, pH=7.1, although the pH at the
field was slightly higher but these algae species are more
tolerant at slightly alkaline conditions; Huss et al. 2002),
and at constant temperature (25°C, similar to conditions at
the field sites) and constant photoperiod (24 h light). The
concentration of microalgae in the added solution did not
exceed the critical value (107cells l−1), above which
filtration and ingestion rates would decrease (Winter
1978). Seawater samples collected from inside the corers
were placed in a dark cool box and immediately taken for
further treatment in the laboratory. Thus, no preservative
was added to the samples; preliminary tests showed that
both formalin and lugol would influence the number of
suspended particles detected by the Coulter counter.
Before removing the corers, all benthic infauna inside
the corer was collected using aquarium nets of 500 μm pore
size. Organisms were identified, their feeding behaviour
determined following Hayward and Ryland (1990), and
biomass measured (dried at 70°C for 3 days). Simpson’s
diversity indices, D (KCS 2001), were estimated for each
habitat type (D=1-Σpi2, where pi is the proportion of
species i in the community; i varies from 1<i<s; s is the
number of species).
Filtration rates were log-transformed and compared
between habitat types by Analysis of Variance (ANOVA)
and Bonferroni tests (Sokal and Rohlf 1995). Benthic
communities (in terms of biomass of filter-feeders) were
compared using multivariate analyses (KCS 2001): Corre-
spondence Analyses (CA), and Cluster Analyses (Euclidean
distance).
Particle Retention Efficiency of the Ria Formosa
Ecosystem
We calculated ingestion rates (IR, particles m−3h−1) by
filter-feeder organisms using established formulae: IR=
(C0-C1)xFL (Hildreth and Crisp 1976) and IR=FRxC1
(Crisp 1984), where C0 is the number of particles in
Fig. 4 Filtration rates
(l m−2h−1) measured in situ at
Ria Formosa lagoon on different
habitat types (* P significant;
tests: ANOVA, Bonferroni; C.I.
is Confidence Interval)
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solution entering a system (particles m−3), C1 is the
number of particles in solution moving out of a system,
FL is water flow (m−3h−1), and FR is filtration rate
(m3h−1). A new model was developed in which C1/C0=
FL/[FR+FL]. This model was applied to estimate particle
retention efficiency, R(%), as R=1-C0/C1=1-FL/[FR=FL]
x100 or R=FR/[FR+FL]x100. Our model was applied to
the Ancão Canal (western portion of Ria Formosa lagoon)
for a 1-m by 1-m section of the canal, using the calculated
filtration rates of local benthic communities, and taking
into consideration the seawater flow in different tidal
regimes. Physical parameters (water depth, current speed)
were registered by a continuous automated data-logger
placed in the canal.
FINE SAND FLAT
SPRING TIDE
FINE SAND FLAT
NEAP TIDE
a e
COARSE SAND FLAT
SPRING TIDE
COARSE SAND FLAT
NEAP TIDE
b f
SEAGRASS BED
SPRING TIDE
SEAGRASS BED
NEAP TIDE
c g
MUD FLAT
SPRING TIDE
MUD FLAT
NEAP TIDE
d h
Fig. 5 Particle retention efficiency (impact) of benthic communities at Ria Formosa lagoon, by habitat type during spring (a–d) and neap (e–h)
tide
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In order to identify which size-classes of suspended
particles were being removed along the channel, seawater
samples were collected (n=3 per site) following a buoy
moving at current speed, both in spring and neap tide, every
0.5 h during two tidal cycles (ebb and flow). Each site
served as a control for next site. Particles were counted by a
calibrated Coulter counter. A further set (n=3) of seawater
samples was collected to identify major planktonic taxa.
Samples destined for particle counting by the Coulter
counter were not treated with preservative solution and
were kept in dark bottles in a cool box, and then treated
within the next few minutes. Samples destined for taxa
identification, were fixed in situ with lugol and kept cold in
the dark. Samples were concentrated in chambers using
sedimentation columns of 50 cm3(Sounia 1981). Particles
were analysed in 5.2 mm2per chamber (n=10), at a
magnification of 400X under an inverted microscope.
Results
Filtration Rates of Benthic Communities from Different
Habitat Types
A total of 55 benthic invertebrate species were
identified in samples from Ria Formosa lagoon, of
which 23 were filter-feeders (Table 1). The maximum
recorded mean standing biomass was 180.2 g ash free dry
mass (AFDM) m−2on seagrass bed areas, where larger
bivalve species such as Donax trunculus, Dosinia
exoleta, Gastrana fragilis, Psammophila magna, Scrobi-
cularia plana, and Solen marginatus occurred. The
lowest diversity indices occurred on mud flats, where
only smaller-sized bivalves such Loripes lacteus, Abra
ovata, and Nucula nucleus, and some polychaetes were
observed. Multivariate analyses showed that benthic
Fig. 6 Absolute frequency of
particles by size class in the
water column, both during flow
(a) and ebb (b) tides, along the
Ancão Canal, Ria Formosa
lagoon
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communities on fine sand flats and seagrass beds of
Cymodocea nodosa were the most similar, with lower
biomass being recorded on either very coarse or very fine
sediments (Fig. 3).
Filtration rates of the sampled benthic communities
ranged from 1.28 to 95.0 lm−2h−1(medians ranged
5.0−45.0 lm−2h−1). Higher values were observed on
seagrass beds and lower values on mud flats (Fig. 4).
This reflected the biomass of filter-feeders at each
sampled habitat, which was positively correlated with
filtration rate (r=0.9 , P<0.01). However, there was no
correlation between biomass of other invertebrate groups
(polychaetes, crustaceans, and gastropods) and filtration
rates.
Particle Retention Efficiency of the Ria Formosa Ecosystem
Based on median filtration rates of benthic communities on
the studied substrate types, our model predicted that
seagrass beds retained 15% of suspended particles during
spring tide and 47% during neap tide (Fig. 5). These
retention efficiencies were much higher than those observed
on mud flats (2.8% during spring tide, and 8% during neap
tide), sand flats of finer grain size (8.8% during spring tide,
and 50% during neap tide), and sand flats of coarser grain
size (7% during spring tide, and 31% during neap tide).
Removal of particles from the water column was higher
during ebb than flow tides, with particles <5 μm diameter
(diatoms, flagellates, and small-sized particulate matter)
Fig. 7 Identification of particles
in the water column, of the most
affected size classes, both during
flow (a) and ebb (b) tide, along
the Ancão Canal, Ria Formosa
lagoon
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being the most commonly removed (Figs. 6–7). Within this
size class, the following taxa were identified: Guinardia
delicatula, Leptocylindricus spp., Chaetocerus spp. and
Pseudonitzschia sp.; Ceratium spp., Dinophysis spp.,
Alexandrium sp., and Gymnodinium sp.
Discussion
Filtration Rates of Benthic Communities from Different
Habitat Types
Filtration rates of benthic communities at Ria Formosa
lagoon were within the range recorded elsewhere (4–375 l
m−2h−1; Dame 1993), but were relatively low with a
recorded maximum of about 100 lm−2h−1on seagrass beds.
Previous studies based their assessments on individual
filtration rates and extrapolated to whole areas based on
bivalve density. However, as demonstrated by Page and
Ricard (1990) doubling bivalve numbers does not neces-
sarily double filtration rates. In fact, complex combinations
of factors must be considered when studying filter-feeding
communities. Bivalves release pseudofaeces and cause
bioturbation, and therefore contribute to any increases in
particle concentration. Refiltration of water previously
cleared of algae can significantly decrease the impacts of
filter-feeding populations on algal populations, and over
time the impacts of filter-feeders should also decrease with
decreased algal concentration. Additionally a carrying
capacity exists for any filtering system (Dame and Prins
1998; Grant et al. 2007), limiting the total amount of
particles that can be retained. Finally, competition among
filter feeders may affect filtration rates (Gunther 1996).
Results from our study demonstrated that seagrass beds
at Ria Formosa lagoon can support higher values of filterer
standing biomass (2.3–180.2 g AFDM m−2yr−1) than
previously estimated, which is in line with studies con-
ducted on soft substrate elsewhere in western Europe
(Table 2). Pihl (1985) describe the importance of vegetated
areas and biogenic structures in promoting filterer biomass
in offshore shallow waters. Several filter-feeders proliferate
in these habitats because they can escape heavy pressure
from bird predation, and thus attain longer life spans and
reach larger body sizes, which is likely to impact their
filtration capacity (Bacher et al. 2003). Exposed flats
typically support lower density and biomass of benthic
organisms than seagrass beds, musselbeds, or algal mats.
This is not only because there is a relationship between
benthic animals and sediment, but also because animal-
animal interactions become important (Mendonça et al.
2007a,b, 2008, 2009).
Particle Retention Efficiency of the Ria Formosa Ecosystem
Our model is relatively simple and can easily be applied to
other coastal ecosystems. One limitation of the model is the
assumption that benthic communities had access to the
entire water column, which is not always the case. Quite
often there is a formation of a benthic boundary layer,
which may have considerable impacts (Fréchette et al.
1993). In situations of laminar flow (sand-flat and mud-flat
areas of Ria Formosa lagoon), benthic communities may
not have access suspended particles outside the boundary
layer (Edwards et al 2005). However, in the presence of
macrophytes, laminar flow does not develop, and this may
contribute to more particles being available to filter-feeders
Table 2 A review on available information on biomass (B) and production (P) of benthic invertebrate macrofauna at several coastal areas in
western Europe (AFDM is ash free dry mass)
AreaHabitat typeBiomass, B
(g AFDM m−2)
Production, P
(gAFDM m−2yr−1)
Reference
Ria Formosa Lagoon, PortugalFine sand flat
Coarse sand flat
Seagrass bed (Zostera noltii)
Soft bottoms
Arenicola marina flat
Nereis-Corophium belt
Seagrass bed (Zostera noltii)
Mud flat
Soft bottoms
Soft bottoms
Arenicola marina flat
Soft bottoms
Soft bottoms
54.2
15.2
16.7
3.2–37.4
27.6
16.5
30.2
4
0.1–119.9
13.2
20.4–30.4
3.8–4.5
2.0–70.0
54.7
32.5
22.4
26.1–118.4
50.2
17.5
48.2
1.7
50.3–57.4
13.3
15.4–20.3
21.0–27.0
23.0–273.0
Sprung 1994
Sprung 1994
Sprung 1994
Anadón 1980
Asmus 1987
Asmus 1987
Asmus 1987
Buchanan and Warwick 1974
Wolff and de Wolff 1977
Warwick and Price 1975
Mendonça 1997
Evans 1983
Pihl 1985
Foz Estuary, Spain
Wadden Sea, Germany
Wadden Sea, The Netherlands
Grevelingen Estuary, The Netherlands
Tamar Estuary, UK
Culbin Sands Lagoon, UK
Gullmar Fjord, Sweden
Gullmar Svik, Sweden
Wetlands (2011) 31:1175–11851183
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Page 12

than in open areas. Furthermore, while benthic filter-feeders
consume planktonic organisms, planktonic growth rate can
compensate for this removal (Lucas et al. 1999).
Filter-feeders impacted only a specific size range
(<5 μm) of planktonic organisms and particles. Langdon
and Newell (1990) reported 100% efficiency for 3.9 μm
microspheres in suspension for Geukensia demissa
bivalves. Our results also showed that numerous smaller
particles were introduced during flood tides, and during
these events the effects of filter-feeders may be less
important. However, during ebb tide, the opposite was
observed; it is likely that the effects of filter-feeders are
most important during ebb tide conditions. However, any
reductions in concentrations of size class <5 μm particles
may also be influenced by sedimentation, not just the
activity of suspension-feeders. Therefore, future studies
should combine investigations on particle retention effi-
ciency with detailed measurements of suspended solids in
seawater, the fates of smaller particles, and the ability of
filter-feeding species to actually retain specific particle size-
classes. Vanderploeg et al. (2001) pointed out that selective
filtration by benthic organisms may indirectly promote
blooms of harmful and non-preferred algae, and may be
involved in demineralization of suspended organic
matter and nutrients, indirectly contributing to planktonic
production.
When comparing our results with those from studies
conducted in San Francisco Bay (Clöern 1982), Bay of
Brest (Hily 1991), and Chesapeake Bay (Bundy 2002;
Cerco 2002), the impact of benthic filter feeding commun-
ties in Ria Formosa appears to be lower, possibly due to the
lack of dense bivalve colonies such as musselbeds or oyster
reefs. Their absence nevertheless was compensated by
higher bivalve biomass on seagrass beds contributing
higher particle retention efficiency on this habitat type.
Acknowledgments
of the German-Portuguese project “Die Biologie der Ria Formosa”
funded by the Ministry of Science and Technology of Germany
(project number 03F0562A). Our special thanks to technician Asmus
Petersen for building the apparatus used to measure filtration rates,
and to Ricardo Mendonça, Sérgio Mendonça, and Ana Correia for
their technical support in the preparation of this document.
This study was conducted within the framework
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