Phytoplankton Loss Research Articles

Internal dynamics of NPZD type ecosystem models

The marine ecosystem is an essential part of the Earth's carbon cycle. Therefore, to create model-based scenarios of the future of the Earth's climate, it is inevitable to consider the relevant processes of this system within numerical simulations. In the last two decades, a wide range of submodels of varying complexity have been established representing the marine ecosystem. Their specific behaviour and characteristic responses to, e.g., external forcings, however, are only merely understood. In this study, we present a mathematical method to assess the fundamental behaviour of such models. Three marine ecosystem models of NPZD type, varying in their formulation of phytoplankton loss and zooplankton loss, are analyzed. Following the qualitative theory of dynamical systems, model specific behaviours as well as similarities of the individual models are revealed. For each model, three equilibria are detected and their stability properties are analyzed. Parameters that significantly affect the model solutions are pointed out. Parameter constellations and initial values that cause characteristic model behaviours are determined and discussed. The results obtained provide the basic, theoretical knowledge which is essentially needed to use models in an effective and appropriate way.

The effect of inorganic nitrogen speciation on primary production in the San Francisco Estuary

We describe the results of a series of 96-h enclosure experiments conducted using water from stations in the northern San Francisco Estuary (SFE) along a gradient in ammonium (NH4) and nitrate (NO3) concentrations. Using dual-labeled 13C/15N tracers, we followed the timing and sequence of primary (carbon, C) production and phytoplankton nitrogen (N) use during experimental phytoplankton blooms. Our results show that diatoms consistently drive the phytoplankton blooms in the enclosures. By tracing both C and N uptake we provide clear evidence that high rates of C uptake are linked to phytoplankton NO3, and not NH4, use. Results from kinetics experiments demonstrated higher specific uptake rates (VMAx) for NO3 compared to NH4 in the SFE. Finally, dissolved inorganic carbon and nutrient drawdown ratios in the enclosures from the chronically high NH4 regions of the SFE were substantially lower than predicted from the Redfield ratio, suggesting suppressed C uptake, in relation to other elemental uptake. Our conceptual model of the DIN interactions that lead to higher primary production and phytoplankton blooms in the SFE suggests that higher rates of primary production that accompany phytoplankton NO3 uptake are sufficient to outpace phytoplankton losses, leading to blooms, compared to the lower rates associated with NH4 uptake (only 20% of that based upon NO3). Historical changes in wastewater practices have increased the proportion of NH4 to the DIN pool in the SFE leading to reduced access to NO3 by phytoplankton. This may help to explain some of the reduced primary production and phytoplankton biomass observed there since the 1970s.

Does warming enhance the effect of microzooplankton grazing on marine phytoplankton in the ocean?

We evaluated a hypothesis derived from the metabolic theory of ecology (MTE) that the ratio of microzooplankton herbivory (m) to phytoplankton growth (μ) will arise in a warming ocean because of the different temperature dependencies of autotrophic and heterotrophic organisms. Using community‐level growth and grazing data from dilution experiments, generalized additive models (GAMs) were constructed to describe the effects of temperature and chlorophyll on m : μ. At low chlorophyll levels, m : μ decreases with increasing temperature, whereas at high chlorophyll levels, m : μ increases initially with temperature before reaching a peak and then declines. These complex responses of m : μ result from mixed effects of temperature and chlorophyll on microzooplankton biomass (Bz), biomass‐specific microzooplankton grazing rate (m : Bz), and phytoplankton growth rate (μ). Bz decreases with rising temperature and increases with rising chlorophyll. m : Bz increases with temperature and decreases with chlorophyll. Nutrient‐enriched growth rate of phytoplankton (μn) and μ increase with increasing temperature and chlorophyll. Holding chlorophyll constant, the calculated activation energies of m : Bz and μn are 0.67 ± 0.05 and 0.36 ± 0.05 eV, respectively, both consistent with previous MTE estimates for heterotrophs and autotrophs. Our study indicates that warming may enhance phytoplankton losses to microzooplankton herbivory in eutrophic but not in oligotrophic waters. The GAM analysis also provides important insights into underlying system relationships and reasons why community‐level responses in natural systems may depart from theory based on laboratory data and individual species.

On a nonlocal reaction-diffusion-advection system modeling phytoplankton growth with light and nutrients

We investigate the steady state solutions of a phytoplankton-nutrient system proposed by Huisman et al. in [14] that models the dynamics of a single phytoplankton species whose growth is limited by light and nutrients in a vertical water column. We first study the existence and nonexistence problem of the model and prove there is at least one positive solution to the system when the parameters involved are in a suitable range. We then analyze the limiting profiles of the positive solutions as the specific phytoplankton loss rate approaches zero and as the diffusion coefficient of the system tends to zero, respectively. In the small diffusion case, we show that the phytoplankton species all die out regardless of how large the nutrient supply is, and the nutrients distribution approaches a linear function determined by the parameters of the system. This phenomenon is in sharp contrast to that of the model studied by Du and Hsu [3, 4], where the phytoplankton species can concentrate at the bottom, at the surface or at a specific depth of the water column decided by the amplitude of the nutrient supply. We also study the asymptotic profile of the positive solution when the diffusion coefficient is very large. Our results reveal that in such a case the phytoplankton and the nutrients distribute evenly in the water column. Our concentration results also reveal that passive diffusion and active movement (sinking or floating) should be in proportion to the oscillation phenomena showed in [14, 24] to occur.

Did the loss of phytoplanktivorous fish contribute to algal blooms in the Mwanza Gulf of Lake Victoria?

Possible causes of the increased algal blooms in Lake Victoria in the 1980s have been disputed by several authors; some suggested a top-down effect by the introduced Nile perch, whereas others suggested a bottom-up effect due to eutrophication. In this article the potential impact is established of grazing by fish on phytoplankton densities, before the Nile perch upsurge and the concomitant algal blooms in the Mwanza Gulf. The biomass and trophic composition of fish in the sublittoral area of the Mwanza Gulf were calculated based on catch data from bottom trawls, and from gill nets covering the whole water column. Estimates of phytoplankton production in the same area were made from Secchi values and chlorophyll concentrations. The total phytoplankton intake by fish was estimated at 230 mg DW m−2 day−1. The daily gross production ranged from 6,200 to 7,100 mg DW m−2 day−1 and the net production from 1,900 to 2,200 mg DW m−2 day−1. Thus, losses of phytoplankton through grazing by fish were about 3–4% of daily gross and 10–12% of daily net phytoplankton production. As a consequence it is unlikely that the phytoplankton blooms in the second half of the 1980s were due to a top-down effect caused by a strong decline in phytoplankton grazing by fish.

Growth and grazing rate dynamics of major phytoplankton groups in an oligotrophic coastal site

There has been more attention to phytoplankton dynamics in nutrient-rich waters than in oligotrophic ones thus requiring the need to study the dynamics and responses in oligotrophic waters. Accordingly, phytoplankton community in Blanes Bay was overall dominated by Prymnesiophyceae, remarkably constant throughout the year (31 ± 13% Total chlorophyll a, Tchl a) and Bacillariophyta with a more episodic appearance (20 ± 23% Tchl a). Prasinophyceae and Synechococcus contribution became substantial in winter ( Prasinophyceae = 30% Tchl a) and summer ( Synechococcus = 35% Tchl a). Phytoplankton growth and grazing mortality rates for major groups were estimated by dilution experiments in combination with high pressure liquid chromatography and flow cytometry carried out monthly over two years. Growth rates of total phytoplankton (range = 0.30–1.91 d −1) were significantly higher in spring and summer ( μ > 1.3 d −1) than in autumn and winter ( μ ∼ 0.65 d −1) and showed a weak dependence on temperature but a significant positive correlation with day length. Microzooplankton grazing (range = 0.03–1.4 d −1) was closely coupled to phytoplankton growth. Grazing represented the main process for loss of phytoplankton, removing 60 ± 34% (±SD) of daily primary production and 70 ± 48% of Tchl a stock. Chl a synthesis was highest during the Bacillarophyceae-dominated spring bloom (Chl a synt = 2.3 ± 1.6 μg Chl a L −1 d −1) and lowest during the following post-bloom conditions dominated by Prymnesiophyceae (Chl a synt = 0.23 ± 0.08 μg Chl a L −1 d −1). This variability was smoothed when expressed in carbon equivalents mainly due to the opposite dynamics of C:chl a (range = 11–135) and chl a concentration (range = 0.07–2.0 μg chl a L −1). Bacillariophyta and Synechococcus contribution to C fluxes was higher than to biomass because of their fast-growth rate. The opposite was true for Prymnesiophyceae.

Recent advances in the Mediterranean researches on zooplankton: from spatial–temporal patterns of distribution to processes oriented studies

In this review we focus on research performed by Italian scientists on pelagic communities, from microzooplankton to micronekton, mainly in the Italian Seas. We considered published data, mostly as grey literature, and unpublished ones. Firstly we describe data collected over a time span of more than 30 years, during several cruises all around the Italian peninsula on zooplankton composition and distribution. We identified rare vs. common species, which enhanced biodiversity of the pelagic ecosystem. Time series, some also very long, allowed us to describe seasonal recurrent patterns, interannual fluctuations and recent shifts driven by climatic changes. More recently Italian researches were processes oriented and we analyzed results obtained on the impact of predation of both micro- and mesozooplankton on both autotrophic and heterotrophic preys. Carbon fluxes through zooplankton components were variable in space and time, but accounted for important phytoplankton losses, and when this resource became scarce they relied on heterotrophic production. Through respiration measurements of mesozooplankton another aspect of the C flux was estimated showing an increase in C demand in the most oligotrophic area. Egg production by copepods appeared to be mostly controlled by temperature and quantity/quality of available food.

Response of the plankton community to herbicide application (triazine herbicide, simetryn) in a eutrophicated system: short-term exposure experiment using microcosms

The methylthiotriazine herbicide, simetryn, is commonly used in Japan, and its concentration in surface water is often high enough to affect natural phytoplankton. To estimate how the plankton community in eutrophic systems respond to short-term exposure of realistic concentrations of simetryn, we collected plankton from a eutrophic lake and exposed them to low (20 μg l−1) and high (100 μg l−1) concentrations of simetryn for 12 days in microcosm tanks (50 l). High concentrations significantly lowered total phytoplankton biomass, particularly green algal density. Consequently, the species composition was severely modified by simetryn application. However, there was no apparent impact of simetryn on microbial food-web components, bacteria, heterotrophic nanoflagellates (HNF), and ciliates. Despite the decreased abundance of algal food, the zooplankton community showed subtle changes with simetryn application. The results indicate that the direct impact of simetryn on planktonic organisms other than phytoplankton, particularly on microbial food-web components, is weak. The indirect impact of simetryn on zooplankton through the change of food quality and quantity was also small. It has been suggested that the persistence of microorganisms, alternative food for zooplankton, probably dilutes the indirect impact of simetryn on zooplankton by compensating for the loss of food phytoplankton. Consequently, the plankton community in eutrophicated systems is resistant to the herbicide at a feasible concentration for a short period of time.

Response of lower trophic organisms to nutrient input and effects on carbon budget: a mesocosm experiment

The roles of heterotrophic organisms (microzooplankton, mesozooplankton, bacteria and heterotrophic nanoflagellates) were examined during a nutrient enrichment experiment using a mesocosm in Saanich Inlet, British Columbia, Canada. Grazing rates of microzooplankton, copepods, and Noctiluca scintillans were respectively estimated by the dilution method, from the egg production, and the apparent growth rate. The primary production increased by about 11 times during the initial 3 days, and the grazing rate by zooplankton also increased by 7.4 times. The primary production exceeded the grazing rate during the initial 5 days, after that, almost balanced rates were observed. Biomass peaks of bacteria and HNFs (heterotrophic nanoflagellates) were observed after the decline of the phytoplankton bloom. Bacterial production and HNF bacterivory gradually increased from the beginning to the end of the experiment. Microzooplankton consistently removed about half of the primary production. The contribution of microzooplankton to grazing was largest during the initial 7 days. Heterotrophic dinoflagellates were the most dominant component of the microzooplankton, but oligotrich ciliates showed the fastest growth response to phytoplankton production. Noctiluca scintillans became an important grazer after the bloom. Overall, the contribution of microzooplankton grazing was the largest of the processes through which phytoplankton were lost. Cell sinking was a minor component contributing to loss of phytoplankton. Thus, oligotrich ciliates and heterotrophic dinoflagellates were the most plausible organisms contributing to the steady state of phytoplankton concentrations.

Seasonal and geographic variations in phytoplankton losses from the mixed layer on the Northwest Atlantic Shelf

The total daily phytoplankton loss from the mixed layer is estimated as the difference between the primary production and the realized change of phytoplankton carbon biomass. A Monte Carlo procedure is used to recover the total loss rates for ten geographic locations on the Northwest Atlantic continental shelf. The strong seasonal and geographic variations in mixed-layer loss rates of phytoplankton are tied closely to the primary production. The daily, mixed-layer, total loss ranges from 50 to 1000 mg C m − 2 d − 1 , which is compared with the output of process models, the closure error being generally less than 10% of the total loss. The model results show that the annual respiration is generally greater than losses due to zooplankton grazing and sinking, except that zooplankton grazing dominates other loss terms on the west Greenland shelf.

The role of wind in determining the timing of the spring bloom in the Strait of Georgia

A coupled biophysical model of the Strait of Georgia (SoG), British Columbia, Canada, has been developed and successfully predicts the timing of the spring phytoplankton bloom. The physical model is a one-dimensional vertical mixing model, using a K-profile parametrization of the boundary layer, forced with high frequency meteorological data. The biological model includes one phytoplankton class (microphytoplankton) and one nutrient source (nitrate). The spring bloom in the SoG occurs when phytoplankton receive enough light that their growth rates exceed their loss rates. The amount of light that the phytoplankton receive is a function of solar radiation and the depth of mixing. The model was used to determine what physical factors are controlling the phytoplankton losses and the light received by the phytoplankton. Wind was found to control the spring bloom arrival time, with strong winds increasing the mixing-layer depth and delaying the bloom. The amount of incoming solar irradiance, through amount of cloud cover, had a secondary effect. The freshwater input (primarily Fraser River discharge) had an insignificant effect on the timing. Increased freshwater flux increases the buoyancy flux and thus decreases the mixing-layer depth but also increases the strength of the estuarine circulation, increasing the advective loss.

Hydrodynamic control of phytoplankton loss to the benthos in an estuarine environment

Field experiments were undertaken to measure the influence of hydrodynamics on the removal of phytoplankton by benthic grazers in Suisun Slough, North San Francisco Bay. Chlorophyll a concentration boundary layers were found over beds inhabited by the active suspension feeders Corbula amurensis and Corophium alienense and the passive suspension feeders Marenzellaria viridis and Laonome sp. Benthic losses of phytoplankton were estimated via both the control volume and the vertical flux approach, in which chlorophyll a concentration was used as a proxy for phytoplankton biomass. The rate of phytoplankton loss to the bed was positively correlated to the bed shear stress. The maximum rate of phytoplankton loss to the bed was five times larger than estimated by laboratory‐derived pumping rates for the active suspension feeders. Reasons for this discrepancy are explored including a physical mechanism whereby phytoplankton is entrained in a near‐bed fluff layer where aggregation is mediated by the presence of mucus produced by the infaunal community.

Modelling the vertical distribution of bromoform in the upper water column of the tropical Atlantic Ocean

Abstract. The relative importance of potential source and sink terms for bromoform (CHBr3) in the tropical Atlantic Ocean is investigated with a coupled physical-biogeochemical water column model. Bromoform production is either assumed to be linked to primary production or to phytoplankton losses; bromoform decay is treated as light dependent (photolysis), and in addition either vertically uniform, proportional to remineralisation or to nitrification. All experiments lead to the observed subsurface maximum of bromoform, corresponding to the subsurface phytoplankton biomass maximum. In the surface mixed layer, the concentration is set by entrainment from below, photolysis in the upper few meters and the outgassing to the atmosphere. The assumed bromoform production mechanism has only minor effects on the solution, but the various loss terms lead to significantly different bromoform concentrations below 200 m depth. The best agreement with observations is obtained when the bromoform decay is coupled to nitrification (parameterised by an inverse proportionality to the light field). Our model results reveal a pronounced seasonal cycle of bromoform outgassing, with a minimum in summer and a maximum in early winter, when the deepening surface mixed layer reaches down into the bromoform production layer.

Estimation of phytoplankton loss rate by remote sensing

A method is presented for estimation of seasonally‐varying, total loss rate of phytoplankton from time series of satellite‐derived phytoplankton biomass data. The loss is calculated as the difference between the (modelled) rate of photosynthesis and the observed, realized rate of change of phytoplankton biomass. A Monte Carlo procedure is used to recover the loss rates. The (biomass‐normalized) total loss rate shows a seasonal cycle with values ranging from 0.5 to 3 mg C (mg Chl)−1 h−1 and shows an abrupt shift during the spring bloom. On the other hand, the absolute loss rate increases during blooms, a consequence of the increase in the biomass. The normalized total loss rate can be further expressed as a time‐varying fraction of the assimilation number. The fraction lies in the range from 0.2 to 0.8. During the increasing (decreasing) phase of phytoplankton blooming, the ratio of growth to total loss increases (decreases), such that this ratio may have value as an ecological indicator for blooms.

Phytoplankton growth conditions during autumn and winter in the Irminger Sea, North Atlantic

Physical and chemical properties of the water column, along with meteorological condi- tions were examined for their relationship with phytoplankton biomass in the Irminger Sea during late autumn and early winter. Data were collected during 2 cruises to the region in November and December 2001 and November 2002. Phytoplankton biomass was approximated by (chl a) concen- trations within the water column. When examined during autumn and winter alone, the Irminger Sea was suitably described as one biogeochemical region responding to varying meteorological forcing. Hydrographic differences within the region were not observed to have a significant effect on phyto- plankton growth during this period. Strong correlations with latitude were seen in chl a concentra- tions, physical conditions (including mixed layer depth) and meteorological forcing (including net heat flux). Variability in autumn/winter phytoplankton growth conditions appears to be driven by light limitation modulated by meteorological forcing. The temporal and spatial scales of locations sampled in 2001 represent a progression in the physical and biological conditions from late autumn to early winter. Along this 'virtual transect', a baseline value of approximately 0.1 mg m -3 is seen in the mean chl a concentrations within the mixed layer. We postulate that convection provides a mech- anism for reduction of net losses of phytoplankton, by helping to keep phytoplankton within the mixed layer. Under such conditions, a deeper and therefore more accurate estimation of the critical depth would be valid. Evidence of the extended maintenance of phytoplankton within the mixed layer is presented in the form of the relative dominances of different phytoplankton groups.

Accounting for grazing dynamics in nitrogen‐phytoplankton‐zooplankton (NPZ) models

Nitrogen‐phytoplankton‐zooplankton (NPZ)‐type models are widely used to explore the dynamics of marine planktonic ecosystems. Within these models, grazing by zooplankton on phytoplankton that are subjected to varying degrees of nitrogen limitation is described using N‐based grazing kinetics together with fixed N assimilation efficiency. There is no empirical evidence for zooplankton displaying such behavior; on the contrary, there is evidence for a decline in zooplankton growth rates on consumption of N‐impoverished prey, with decreased assimilation efficiencies coupled with decreased ingestion rates of nutrient‐limited (i.e., poor quality, low N: C) prey. Unwittingly, then, traditional NPZ models make unjustified assumptions concerning changes in predator‐prey interactions on consumption of low‐quality prey. We explore the effects of this flawed description, also asking why NPZ models can still give reasonable descriptions of field data. Our conclusion is that one flaw may be countered by another, namely, by an inadequate description of nongrazing phytoplankton losses. In nature, these nongrazing losses are enhanced within nutrient‐depleted phytoplankton populations. In models, a decline in grazing losses on nutrient‐deprived phytoplankton is, de facto, compensated for by enhanced nongrazing losses. While the fit of the revised model to the data is not dissimilar to that of the original model (with its inappropriate descriptions of grazing and nongrazing phytoplankton mortality), the fate of primary production is very different. With the biologically more acceptable description, more material flows through the detrital compartment, with important implications for trophic dynamics. Care must be taken not to oversimplify descriptions of biology in models, as these may directly or indirectly mask the simulation of other important ecological processes.

Tidal exchange, bivalve grazing, and patterns of primary production in Willapa Bay, Washington, USA

Willapa Bay, Washington, USA, is a shallow, coastal-plain, upwelling-influenced estuary where Pacific oysters Crassostrea gigas are intensively cultivated. CTD transect data show that in the long-term average over the May to September growing season, Willapa Bay is a sink for oceanic phytoplankton, not a net exporter: as the tidal circulation stirs ocean water into the estuary, chlorophyll concentration declines by 30 to 60% relative to a hypothetical dilution of the ocean end-member. A 3D circulation model (General Estuarine Transport Model, GETM) was modified to include a phyto- plankton-like tracer subject to variable intertidal benthic grazing rates. The grazing rate that best reproduces the along-estuary phytoplankton biomass profile agrees, within confidence limits, with an estimate of cultivated-oyster filtration from literature values. Oysters and other intertidal benthic grazers may thus be the primary cause of the net loss of phytoplankton within the estuary in summer. These grazers appear to be within a small factor of their carrying capacity: as bay-total filtration capacity is increased in the model, the chlorophyll intrusion shortens and food intake per individual grazer declines. Nevertheless, only 8 to 15% of the net tidal supply of oceanic phytoplankton is consumed within the estuary. Even in this well-flushed system, the small-scale structure of tidal transport—rather than total oceanic supply—controls overall food availability for the benthos.

Turbulent mixing and phytoplankton spring bloom development in a deep lake

A one‐dimensional (1‐D) mechanistic phytoplankton model combined with a 1‐D hydrodynamic model was applied to simulate phytoplankton growth during winter and spring in deep monomictic Upper Lake Constance. Modeled chlorophyll a concentrations agree well with data from the 8‐y time period considered. In particular, the interannual variation in the timing of phytoplankton growth is adequately simulated by the model. The onset of phytoplankton blooms in Upper Lake Constance is not sensitive to variations in the photosynthetically active radiation, the sinking velocity of the algae, or the effect of water temperature on biological process rates, but is primarily determined by turbulent diffusion (i.e., by the transition from strong mixing in winter and early spring to weak mixing). The transition in mixing conditions and thus also the beginning of phytoplankton population growth correlates with the build up of the first slight temperature stratification. Simulations performed without consideration of phytoplankton loss due to grazing overestimate algal biomass soon after the onset of algal growth. Including grazing by zooplankton substantially improves the agreement between model and data and suggests that ciliate grazing in particular leads to a significant reduction of phytoplankton abundance in spring after the start of the algal bloom.

Variability in phytoplankton pigment biomass and taxonomic composition over tidal cycles in a salt marsh estuary

Tidal flow causes high temporal variability in environmental properties that impact ecosystem dynamics. Microbes such as phytoplankton are especially susceptible to tidal advection and mixing, and understanding their role in estuarine food webs and biogeochemical cycles requires information on their biomass and taxonomic composition over short time scales (e.g. tidal cycles). We conducted a survey of phytoplankton pigment biomass and taxonomic composition over complete tidal cycles in 2 salt marsh creeks on 5 sampling occasions from July to September 2000, and assessed environmental factors regulating phytoplankton properties. Tidal input of low chl a water combined with phytoplankton losses (microzooplankton grazing, oyster grazing, settling) caused large decreases in phytoplankton biomass (by 47 to 51% on average) on the flood tide, and also influenced the taxonomic composition. Depending on sampling date, pennate diatoms or flagellates were pri- marily reduced on the flood tide. One sampling date followed a heavy rain event, and was marked by substantial increases in tidal creek nutrient concentrations and reduced microzooplankton grazing rates, emphasizing the need to consider the combined influences of nutrients and grazing in explain- ing bloom formation following rain events. The high tidal variability in phytoplankton properties sug- gests that strict attention to tidal phase is needed in determining long-term trends or inter-estuary comparisons in phytoplankton biomass, and primary production in tidally-driven estuaries.

Relative effects of physical and biological processes on nutrient and phytoplankton dynamics in a shallow estuary after a storm event

The effects of advection, dispersion, and biological processes on nitrogen and phytoplankton dynamics after a storm event in December 2002 are investigated in an estuary located on the northern New South Wales coast, Australia. Salinity observations for 16 d after the storm are used to estimate hydrodynamic transports for a one-dimensional box model. A biological model with nitrogen limited phytoplankton growth, mussel grazing, and a phytoplankton mortality term is forced by the calculated transports. The model captured important aspects of the temporal and spatial dynamics of the bloom. A quantitative analysis of hydrodynamic and biological processes shows that increased phytoplankton biomass due to elevated nitrogen loads after the storm was not primarily regulated by advection or dispersion in spite of an increase in river flow from <1 to 928×103 m3 d−1. Of the dissolved nitrogen that entered the surface layer of the estuary in the 16 d following the storm event, the model estimated that 28% was lost through exchange with the ocean or bottom layers, while 15% was removed by the grazing of just one mussel species,Xenostrobus securis, on phytoplankton, and 50% was lost through other biological phytoplankton loss processes.X. securis grazing remained an important loss process even when the estimated biological parameters in the model were varied by factors of ± 2. The intertidal mangrove pneumatophore habitat ofX. securis allows filtering of the upper water column from the lateral boundaries when the water column is vertically stratified, exerting top-down control on phytoplankton biomass.