Description and use of an improved method for determining estuarine zooplankton grazing rates on phytoplankton

A method of rapidly determining zooplankton grazing rates on natural mixed phytoplankton populations using ¹⁴C is described. The method simplifies the design of grazing experiments as the grazing time can be kept short enough to prevent recycling of the isotope, and growth of the phytoplankton substrate. Very high specific activity, ¹⁴C-labelled phytoplankton concentrated either by centrifugation or sieving, may be used either as the sole grazing substrate, or as a tracer in natural mixed phytoplankton. Zooplankton, confined in glass jars at either ambient, or higher than ambient concentrations, are permitted to feed on the phytoplankton for periods of 30 min and 2 h, and are then separated by sieving. The zooplankton community grazing rate, or, if the samples are sorted into species, the individual species grazing rates, can be determined after scintillation counting of the zooplankton. The rate of appearance of ¹⁴C-labelled phytoplankton in the zooplankton is an estimate of the grazing rate, and the slope of the line joining the grazing rates at various phytoplankton concentrations gives an estimate of the grazing rate constant for the zooplankton population. The method provides a quick way of obtaining both zooplankton population, and individual species grazing rates on natural mixed phytoplankton. In two experiments, labelled phytoplankton was used as the sole grazing substrate in concentrations ranging between 0.4 and 5 times ambient levels. Grazing rate constants, for net-caught zooplankton concentrated to 46 times (Experiment 1) and 28 times (Experiment, 2) ambient estuarine levels were-0.14and-0.12 of the phytoplankton standing stock per day, respectively. There was a linear increase in the amount of phytoplankton grazed with an increase in phytoplankton concentration up to four times ambient phytoplankton levels. When tracer amounts of labelled phytoplankton were added to samples containing both phytoplankton and zooplankton at ambient concentrations the grazing rate constants were-0.28 and-0.42 of the phytoplankton standing stock per day. We conclude that zooplankton grazing was the major control factor of phytoplankton population size during October–November 1975 in South West Arm, Port Hacking, near Sydney, Australia.     

Dependence of phytoplankton production on rhythm and rate of elimination

This paper investigates the dependence of phytoplankton production upon rhythm and rate of zooplankton grazing and presents a mathematical model for calculating the most important parameters. Both uniform and non-uniform grazing are described mathematically. Non-uniform grazing, expressed by a sinusoidal curve, is usually found in bathyplanktonic ecosystems with migratory consumers. Phytoplankton production depends on the time of grazing; the nearer grazing occurs toward nightfall, the higher is the phytoplankton production. In order to calculate phytoplankton productivity and the amount of food consumed by the zooplankton, experimental data on generation time of phytoplankters, their mortality rates, initial and final standing stocks, and information on diurnal grazing rhythms must be available. If the distribution of grazing rates is sinusoidal and mortality rate constant, the equations presented allow the calculation of phytoplankton productivity with an error of about 6%.     

A two-dimensional numerical model for eutrophication in Baiyangdian Lake

Hydrodynamic, physical, and biochemical processes in the Baiyangdian Lake water environment were analyzed comprehensively. An eutrophication ecodynamics model including the effects of reed resistance on flow was coupled with the hydrodynamics governing equations. An improvement on the Water Quality Analysis Simulation Program (WASP, a modeling system introduced by the US Environmental Protection Agency) is established, which uses the zooplankton kinetic equation. The model simulates water quality constituents associated with eutrophication in the lake, including phytoplankton, zooplankton, nitrogen, phosphorus, dissolved oxygen, and others. Various kinetic coefficients were calibrated using measured data or information from relevant literature, to study eutrophication in the lake. The values calculated by the calibrated model agree well with field data, including ammonia nitrogen, total nitrogen, total phosphorus and dissolved oxygen. Changes related to nutrition and dissolved oxygen during the processes were simulated. The present model describes the temporal variation of water quality in Baiyangdian Lake with reasonable accuracy. Deviations between model-simulated and observed values are discussed. As an ideal tool for environmental management of the lake, this model can be used to predict its water quality, and be used in research to examine the eutrophication process.     

The interplay between environmental conditions and allee effects during the recovery of stressed zooplankton communities

Many important ecological phenomena depend on the success or failure of small introduced populations. Several factors are thought to influence the fate of small populations, including resource and habitat availability, dispersal levels, interspecific interactions, mate limitation, and demographic stochasticity. Recent field studies suggest that Allee effects resulting from mate limitation can prevent the reestablishment of sexual zooplankton species following a disturbance. In this study, we explore the interplay between Allee effects and local environmental conditions in determining the population growth and establishment of two acid-sensitive zooplankton species that have been impacted by regional anthropogenic acidification. We conducted a factorial design field experiment to test the impact of pH and initial organism densities on the per capita population growth (r) of the sexual copepod Epischura lacustris and the seasonally parthenogenetic cladoceran Daphnia mendotae. In addition, we conducted computer simulations using r values obtained from our experiments to determine the probability of extinction for small populations of acid-sensitive colonists that are in the process of colonizing recovering lakes. The results of our field experiment demonstrated that local environmental conditions can moderate the impacts of Allee effects for E. lacustris: Populations introduced at low densities had a significantly lower r at pH 6 than at pH 7. In contrast, r did not differ between pH 6 and 7 environments when E. lacustris populations were introduced at high densities. D. mendotae was affected by pH levels, but not by initial organism densities. Results from our population growth simulations indicated that E. lacustris populations introduced at low densities to pH 6 conditions had a higher probability of extinction than those introduced at low densities to a pH 7 environment. Our study indicates that environmental conditions and mate limitation can interact to determine the fate of small populations of sexually reproducing zooplankton species. If a more rapid recovery of acid-damaged zooplankton communities is desired, augmentation of dispersal levels may be needed during the early phases of pH recovery in order to increase the probability of establishment for mate-limited zooplankton species.     

Variability and its effect on the 24h chlorophyll budget of a small marine basin

In an intensive study (lasting 25 h) of the production, export and grazing of phytoplankton in a small marine basin, it was found that 58% of the production (11% of the total standing stock) was lost by exchange with the sea and 34% was consumed by grazing of zooplankton. The measured production of phytoplankton could be balanced, to within a few percent, against grazing, export, and a small, measured, net change in the total standing stock of the basin. Large variations were observed in concentrations of chlorophyll and zooplankton at the mouth of the basin over the 25 h period. These variations were associated with changes in the height of the tide, but were about 2^1/2 h out of phase with it. Strong negative correlations were observed between chlorophyll and transport, such that only 35% of the chlorophyll exported was exchanged via the mean flow, while 65% was exchanged via the fluctuations. The correlation was even more striking with zooplankton, for which virtually all the export was associated with the fluctuations in the transport. Time series observations in the centre of the basin revealed considerable short-term variability in both chlorophyll and zooplankton, but the variations were smaller than those observed at the mouth of the basin, and the phase lag with the tide was longer. The variability studies enable suggestions to be made about more economical design of sampling programs, but illustrate the difficulty of providing verification data for any continuous model of primary production in such a basin.Bedford Institute Contribution No. 231.Canadian Contribution to IBP No. 97.     

Influence des conditions hivernales sur les productions phyto-et zooplanctoniques en Méditerranée Nord-Occidentale. V. Biomasse et production zooplanctonique — relations phyto-zooplancton

The cruise “Mediprod I” of the R.V. “Jean Charcot” covered an area of the Mediterranean Sea situated North of the 40th parallel and West of Corsica; during two 15-day legs, the first when Winter conditions ended, in March, the second in Spring conditions, in April, a 48-station network was surveyed as to primary and secondary production, as well as hydrological conditions. The first survey revealed a higher concentration of zooplankton in hydrologically stable areas, especially in the South-East, although zooplankton values were rather low throughout the whole area surveyed. The total zooplankton per unit surface was lower at the edges of the surveyed area, where phytoplankton was scarce. An increase in zoo-plankton biomass was observed between the two surveys, mostly in the central areas and near the surface. An important difference between both legs is in the proportion of organisms of different sizes collected by the Clarke-Bumpus sampler: the-200 μ:+200 μ organisms ratio, which is around 1 during the first leg, is much lower during the second leg. Two methods were used in estimating the biomass: the +200 μ fraction of a Clarke-Bumpus sample (Cl-B towed with a 50-μ net fitted on the sampler) was collected by sieving the sample through a 200-μ mesh nylon; standard vertical WP 2 hauls were performed (200 μ mesh). Both roughly show the same zooplankton (weight per unit surface) distribution pattern. However, higher estimations of the total biomass of the volume investigated were sometimes provided by the Clarke-Bumpus method during the first leg, probably due to the distribution of the animals according to their size-classes. A graph of chlorophyll versus zooplankton for surface waters suggests that zooplankton has a limiting effect on the development of phytoplankton in April only. Apparent growth rates of cooplankton are less than those for populations in the laboratory or enclosed environments. Values of the mean secondary production vary from 18 mg C.m^-2.day^-1 for the first leg, to 230 mg C.m^-2.day^-1 for the second leg. Estimations of net efficiency for energetic transfer between phyto- and zooplankton lie between 7 and 26%. As far as our hypotheses as regards physiological coefficients are valid, we can assume that the effect of grazing relative to primary production is greater in the border areas than in the central area, thus increasing the contrast between both areas with time. Phosphorus excretion rate by zooplankton seems to be less than that measured in the Atlantic Ocean. We suggest that in Spring zooplankton excretion is not the main phosphorus recycling process. Primary productivity measurements, apparent growth coefficients, and estimated grazing rate have been used to calculate the expected mean biomass per unit area during the second leg. A 7% loss of phyto- and zooplankton from the upper 100-m layer must be assumed to explain the observed biomass variation. Vertical mixing and sinking of surface water on an isopycnal slope are responsible for such loss, which can also affect, to an undetermined degree, the phosphorus stock introduced into the surface layer by the Mediterranean deep-water formation mechanism. We suggest that nearly half the total loss of phytoplankton from March to April is attributable to animal grazing.     

浮游动物在生物操纵法除藻中的作用研究

采用室外现场实验和室内受控生态系统实验相结合的方法,研究了富营养化水体中浮游动物对藻类生长的控制作用。对浮游动物与藻类的计数与测量数据,利用SPSS统计软件进行了方差分析(LSD多重比较法)和相关性分析。实验结果显示,总磷含量比总氮含量对浮游动物生长的影响更大。浮游动物与藻类之间呈现显著或极显著相关关系,说明浮游动物群体,尤其甲壳类群体,在适当条件下对藻类群体以及其中的蓝藻有一定的控制作用。适当的鱼类密度下,浮游动物能够起有效的控藻作用,但鱼类密度过高会抑制该作用。底泥在生态系统中起重要作用,能够影响浮游动物对藻类的摄食力。     

Mathematical model and computer simulation of the population dynamics of zooplankton in lake and estuary ecosystems

A mathematical model is developed for the population dynamics and survival probabilities of zooplankton in lake and estuarine. Basic digital computer simulation methodology is used for systematic application of the mathematical model for analysis of the population dynamics of zooplankton. In the analysis of this theoretical modelling, attention is focused on the production and destruction processes of the animal. Both processes are assumed stationary. The birth (production) rate is constant and life-span distribution is independent of time. It is also assumed that the different zooplankton species are independent and share a common aquatic ecosystem, and what happens to one of them does not influence the fate of any other in this system.     

Habitat specialization and the exploitation of allochthonous carbon by zooplankton

The significance of spatial subsidies depends on consumer resource interactions in the recipient habitat. Lakes are subsidized by terrestrial carbon sources, but the pathways of allochthonous carbon through lake food webs are complex and not well understood. Zooplankton vertically partition resources within stratified lakes in response to life history trade-offs that are governed by predators, the quantity and quality of food, and abiotic conditions (e.g., UV, temperature, and viscosity). We measured habitat specialization of zooplankton in an oligotrophic lake where allochthonous and autochthonous resources varied with depth. During stratification, the quantity and quality of zooplankton food was highest in the hypolimnion. We used a yearlong time series of the delta13C of zooplankton and particulate organic matter (POM) to determine which zooplankton species exploited hypolimnetic rather than epilimnetic resources. Because the delta13C of POM decreased with depth, we used the delta13C of zooplankton to detect inter- and intraspecific variation in habitat selection. We incubated Daphnia pulex at discrete depths in the water column to confirm that the delta13C of zooplankton can indicate habitat specialization. Zooplankton that specialized in the epilimnion relied more on allochthonous carbon sources than those that specialized in the hypolimnion. Therefore, the fate of allochthonous carbon subsidies to lakes depends on spatially explicit consumer-resource interactions.     

Trajectories of zooplankton recovery in the Little Rock Lake whole-lake acidification experiment.

Understanding the factors that affect biological recovery from environmental stressors such as acidification is an important challenge in ecology. Here we report on zooplankton community recovery following the experimental acidification of Little Rock Lake, Wisconsin, USA. One decade following cessation of acid additions to the northern basin of Little Rock Lake (LRL), recovery of the zooplankton community was complete. Approximately 40% of zooplankton species in the lake exhibited a recovery lag in which biological recovery to reference basin levels was delayed by 1-6 yr after pH recovered to the level at which the species originally responded. Delays in recovery such as those we observed in LRL may be attributable to "biological resistance" wherein establishment of viable populations of key acid-sensitive species following water quality improvements is prevented by other components of the community that thrived during acidification. Indeed, we observed that the recovery of species that thrived during acidification tended to precede recovery of species that declined during acidification. In addition, correspondence analysis indicated that the zooplankton community followed different pathways during acidification and recovery, suggesting that there is substantial hysteresis in zooplankton recovery from acidification. By providing an example of a relatively rapid recovery from short-term acidification, zooplankton community recovery from experimental acidification in LRL generally reinforces the positive outlook for recovery reported for other acidified lakes.     

Hydrodynamic and towing characteristics of a modified model of the Clarke Jet Net

In a series of studies to develop an effective high speed plankton sampler, hydrodynamic research was carried out on a modified model of the Clarke Jet Net. A theoretical analysis of the motion of the sampler was made using an IBM 7040 computer, and test tows were made in an experimental ship tank and at sea. To maintain the roll angle of the sampler at approximately zero, a v-shape bridle was applied. A pair of towing holes should be selected for the particular range of towing velocity. The calculated longitudinal stability seemed good in all the cases examined. Towing tests in the ship tank at 2 to 3 m/sec revealed sufficient stability. The motion of the sampler at speeds higher than 3 m/sec can be assumed from hydrodynamic coefficients derived from the ship tank experiment. Towing-cable characteristics, assumed by computation, indicated that about 500 m at 6 kt and 1,000 m at 12 kt should be paid out to lower the sampler down to 100 m. The effect of the sampler on the cable is found only adjacent to it and is almost negligible beyond 50 m from it in depth. Results of sea tests at 6 and 9 kt confirmed the stability of the sampler. Examination of the catch suggested that the present model can be used, with some modifications, for the quantitative and qualitative sampling of large zooplankton.Contribution No. 231 from the Seikai Regional Fisheries Research Laboratory, Nagasaki.     

Large nonlethal effects of an invasive invertebrate predator on zooplankton population growth rate

We conducted a study to determine the contribution of lethal and nonlethal effects to a predator's net effect on a prey's population growth rate in a natural setting. We focused on the effects of an invasive invertebrate predator, Bythotrephes longimanus, on zooplankton prey populations in Lakes Michigan and Erie. Field data taken at multiple dates and locations in both systems indicated that the prey species Daphnia mendotae, Daphnia retrocurva, and Bosmina longirostris inhabited deeper portions of the water column as Bythotrephes biomass increased, possibly as an avoidance response to predation. This induced migration reduces predation risk but also can reduce birth rate due to exposure to cooler temperatures. We estimated the nonlethal (i.e., resulting from reduced birth rate) and lethal (i.e., consumptive) effects of Bythotrephes on D. mendotae and Bosmina longirostris. These estimates used diel field survey data of the vertical gradient of zooplankton prey density, Bythotrephes density, light intensity, and temperature with growth and predation rate models derived from laboratory studies. Results indicate that nonlethal effects played a substantial role in the net effect of Bythotrephes on several prey population growth rates in the field, with nonlethal effects on the same order of magnitude as or greater (up to 10-fold) than lethal effects. Our results further indicate that invasive species can have strong nonlethal, behaviorally based effects, despite short evolutionary coexistence with prey species.     

Food utilisation of pelagic mysids, Mysis mixta and M. relicta, during their growing season in the northern Baltic Sea

The dietary habits of the pelagic mysid Mysismixta were studied during its growing season at an open sea location in the Gulf of Finland, the northern Baltic Sea. Stomach samples were taken twice a month from June to September 1997. The most abundant phytoplankton taxa in the stomachs were diatoms and dinoflagellates, and copepods and cladocerans were the most abundant zooplankton identified. A clear change was found in the diets during the study period. Small mysids (3 to 6 mm) fed on sedimented phytoplankton in the early summer (90% benthic particles in June) but shifted gradually to a more pelagic and carnivorous diet (>40% pelagic particles, consisting of ca. 60% zooplankton in September). Seasonal changes in mysid capture ability as well as food availability were suggested to affect the diet composition of mysids during their growth. The ratio of pelagic and benthic food particles could – irrespective of the season – be explained by mysid size, whereas the zooplankton:phytoplankton ratio was better explained by season. The stomach analysis suggests that the mysids needed to attain a threshold size of 8 to 11 mm to initiate feeding on the more evasive copepods. Mysids also started to grow faster at the same time as the proportion of copepods increased in the diet, which suggests that copepods are an important energy source for M. mixta in late summer. Finally, a comparison was made between the M. mixta diet and that of the less abundant M. relicta. The diets of the two pelagic mysid species overlapped by 75% (Schoener's index). The main difference was due to M. mixta eating more zooplankton and pelagic material than M. relicta. Received: 15 September 1999 / Accepted: 18 January 2000     

Density-gradient centrifugation as an aid to sorting planktonic organisms. I. Gradient materials

Fish eggs and larvae can be separated from invertebrate zooplankton by isopycnic centrifugation in gradients of sucrose or silica. Preserved samples of invertebrate zooplankton, fish eggs, and fish larvae, representing a typical assortment of marine plankton, were layered over linear gradients of 25 to 60% w/w (weight/weight) sucrose or 0 to 15% w/w silica (as Ludox AM) in 100 c³ swinging buckets, and centrifuged for 1 h at 1000 rpm (revs per minute). In sucrose gradients, the invertebrate zooplankton were confined to the two ends of the gradient, while 85% of the fish eggs were recovered from an intermediate zone (27.5 to 55% w/w). In Ludox AM, the fish eggs banded in a narrow region between 2 and 3% w/w, while fish larvae banded at the bottom of the gradient between 10 and 14% w/w. Of the 6 dominant classes of zooplankton, only Salpa overlapped appreciably with the fish eggs and none overlapped with the fish larvae. Of the gradient materials tested, Ludox AM offers the most advantages; sucrose may also be useful for subfractionation. Gradients of sodium bromide and dextran have been found to be totally unsuitable.Paper of the Journal Series, New Jersey Agricultural Experiment Station, Rutgers University, New Brunswick, New Jersey 08903, USA.     

A box model of carrying capacity for suspended mussel aquaculture in Lagune de la Grande-Entrée,Iles-de-la-Madeleine,Québec

An object-oriented model of environment–mussel aquaculture interactions and mussel carrying-capacity within Lagune de la Grande-Entrée (GEL), Iles-de-la-Madeleine, Québec, was constructed to assist in development of sustainable mussel culture in this region. A multiple box ecosystem model for GEL tied to the output of a hydrodynamic model was constructed using Simile software, which has inherent ability to represent spatial elements and specify water exchange between modelled regions. Mussel growth and other field data were used for model validation. Plackett–Burman sensitivity analysis demonstrated that a variety of bioenergetic parameters of zooplankton and phytoplankton submodels were important in model outcomes. Model results demonstrated that mussel aquaculture can be further developed throughout the lagoon. At present culture densities, phytoplankton depletion is minimal, and there is little food limitation of mussel growth. Results indicated that increased stocking density of mussels in the existing farm will lead to decreased mass per individual mussel. Depending on the location of new farm emplacement within the lagoon, implementation of new aquaculture sites either reduced mussel growth in the existing farm due to depletion of phytoplankton, or exhibited minimum negative impact on the existing farm. With development throughout GEL, an excess of phytoplankton was observed during the year in all modelled regions, even at stocking densities as high as 20 mussels m⁻³. Although mussels cultured at this density do not substantially impact the ecosystem, their growth is controlled by the flux of phytoplankton food and abundance of zooplankton competitors. This model provides an effective tool to examine expansion of shellfish farming to new areas, balancing culture location and density.     

Evaluating the ecological benefits of wildfire by integrating fire and ecosystem simulation models

Fire managers are now realizing that wildfires can be beneficial because they can reduce hazardous fuels and restore fire-dominated ecosystems. A software tool that assesses potential beneficial and detrimental ecological effects from wildfire would be helpful to fire management. This paper presents a simulation platform called FLEAT (Fire and Landscape Ecology Assessment Tool) that integrates several existing landscape- and stand-level simulation models to compute an ecologically based measure that describes if a wildfire is moving the burning landscape towards or away from the historical range and variation of vegetation composition. FLEAT uses a fire effects model to simulate fire severity, which is then used to predict vegetation development for 1, 10, and 100 years into the future using a landscape simulation model. The landscape is then simulated for 5000 years using parameters derived from historical data to create an historical time series that is compared to the predicted landscape composition at year 1, 10, and 100 to compute a metric that describes their similarity to the simulated historical conditions. This tool is designed to be used in operational wildfire management using the LANDFIRE spatial database so that fire managers can decide how aggressively to suppress wildfires. Validation of fire severity predictions using field data from six wildfires revealed that while accuracy is moderate (30-60%), it is mostly dictated by the quality of GIS layers input to FLEAT. Predicted 1-year landscape compositions were only 8% accurate but this was because the LANDFIRE mapped pre-fire composition accuracy was low (21%). This platform can be integrated into current readily available software products to produce an operational tool for balancing benefits of wildfire with potential dangers.     

In search of quorum effects in metacommunity structure: species co-occurrence analyses

Alternative models of community assembly emphasize regional, stochastic, dispersal-based processes or local, deterministic, niche-based processes. Community ecology's historical focus on local processes implicitly assumes that local processes surpass regional processes over time or across space to derive nonrandom metacommunity structure (i.e., a quorum effect). Quorum effects are expected late in succession among nearby sites, whereas quorum effects are not expected early in succession among distant sites. I conducted a meta-analysis of zooplankton data sets encompassing time scales of one to thousands of years and spatial scales of <1 m to thousands of kilometers. Species co-occurrence analyses statistically evaluated presence/absence patterns relative to random patterns obtained with Monte Carlo null models. A series of weighted analyses was conducted and alternative randomization algorithms and null models were evaluated. Most zooplankton metacommunities were randomly structured in unweighted analyses, and the distribution of significant structure did not follow quorum effect predictions. Weighted analyses (e.g., by habitat area) revealed significant, nonrandom structure in most zooplankton metacommunities, but the distribution of significant structure still did not adhere well to quorum effect predictions. Finally, additional weighting for study scale (number of sites) nullified most significant area-weighted structure, and again, the distribution of significant structure did not follow quorum effect predictions. Overall, a quorum effect was not supported, perhaps related to zooplankton life histories and energetics and/or the quorum effect itself. Results at the presence/absence level of resolution indicated that local processes did not generally override regional processes over time or across space to drive community structure. A full integration of dispersal- and niche-based concepts in metacommunity dynamics will be most fruitful for unraveling community assembly. Species co-occurrence analyses were scale dependent (habitat area and study size). Future analyses should use weights for important factors (e.g., habitat area), and meta-analyses should include study scale as an additional factor contributing to apparent patterns.     

Apparent predation on ichthyoplankton by zooplankton and fishes in nearshore waters of southern California

Plankton collected at discrete depths in Santa Monica Bay, California, USA, during January 1982 were examined for fish eggs and larvae that had been attacked or consumed by zooplankton. The bongo net remained open for only 3 min and samples were preserved within 5 min of capture. Juvenile and adult fishes that had been captured by otter trawl and preserved within 20 min of capture were examined for ingested fish eggs and larvae. Three copepods (Corycaeus anglicus, Labidocera trispinosa, and Tortanus discaudatus), one euphausid larva (Nyctiphanes simplex), one amphipod (Monoculoides sp.), and an unidentified decapod larva were found attached to fish larvae in the preserved plankton samples (attachment to 23% of the fish larvae was observed in one sample). Overall, about 5% of the white croaker (Genyonemus lineatus) larvae and 2% of the northern anchovy (Engraulis mordax) larvae had attached zooplankton predators. Most fish larvae with attached zooplankton predators were small. We found no indication of zooplankton predation on fish eggs. Few fish eggs and larvae were found in the digestive tracts of juvenile or adult fishes, and the ingested fish larvae were relatively large. The discussion considers apparent preyspecificity of the zooplankton predators as well as potential biases that may be associated with preserved samples collected by nets.     

Three-dimensional modeling of the lower trophic levels in the Ria de Aveiro (Portugal)

The water and the ecosystem dynamics of the Ria de Aveiro, a shallow, multi-branch lagoon located on the northwest coast of Portugal, are simulated using a new fully coupled 3D modeling system. This model couples the hydrodynamic model SELFE (semi-implicit Eulerian-Lagrangian finite element) and an ecological model extended from EcoSim 2.0 to represent zooplankton dynamics. The model application is based on an unstructured grid spatial discretization, which is particularly appropriate for this system given its complex geometry. The baroclinic circulation is calibrated and validated for different environmental conditions, leading to velocity errors smaller than 5 cm/s across the lagoon. Ecological simulations, focused on zooplankton dynamics represented by a site-specific formulation, are then presented and compared against field data for two contrasting environmental conditions: Autumn 2000 and Spring 2001. Results show that the fully coupled model is able to reproduce the dynamics of the ecosystem in the Spring 2001, fitting the model results inside the range of data variation. During this period zooplankton differences between data and model results are of about 0.005 mg C/l (60%), while other ecological tracers’ differences are generally smaller than 20–30% along the several branches of the lagoon. In the Autumn 2000, the model tends to overestimate zooplankton by a factor of 10 and to underestimate phytoplankton and ammonium, with discrepancies of about 0.1 mg C/l and 4.8 μmol N/l, respectively. Factors like the ecological conditions imposed at the boundaries, the input parameters of the ecological model and the simplification of the ecosystem structure, since phytoplankton is the only primary producer considered, may explain the observed differences.     

Effects of buoyancy, transparency and zooplankton feeding on surface maxima and deep maxima: Comprehensive mathematical model for vertical distribution in cyanobacterial biomass

Inhomogeneous vertical distributions of the cyanobacterial biomass are widely observed during the summer season in stratified lake ecosystems. Among these are surface maxima characterized by surface scum formation and deep or subsurface maxima also known as deep chlorophyll maxima (DCM). The former occurs at the epilimnion in eutrophic lakes, and are usually caused by colonial cyanobacteria such as Microcystis. On the other hand, the latter occurs at the metalimnion and the upper part of the hypolimnion near the thermocline in oligotrophic lakes, and are referred to filamentous cyanobacteria such as Oscillatoria. The aim of this paper is to present a simple mathematical model that can simultaneously describe these phenomena including the annual and diurnal variations, emphasizing the roles of buoyancy regulation, transparency of the lake and zooplankton feeding on cyanobacteria. According to our computer analyses, the increased buoyancy, the low clarity of the lake and the low rate of zooplankton feeding take significant roles in formation of surface maxima, while the reversal of these factors makes deep maxima predominant. Our two-component model with nutrients and cyanobacteria can distinguish between two phenomena by changing the parameters for these factors, without altering the model itself.