Towards a correct description of zooplankton feeding in models: Taking into account food-mediated unsynchronized vertical migration

Complex nature of foraging behaviour of zooplankton makes it difficult to describe adequately zooplankton grazing in models with vertical space. In mean-field models (based on systems of PDEs or coupled ODEs), zooplankton feeding at a given depth is normally computed as the product of the local functional response and the zooplankton density at this depth. Such simplification is often at odds with field observations which show the absence of clear relationship between intake rates of organisms and the ambient food density. The observed discrepancy is generic and is often caused by fast non-synchronous vertical migration of organisms with different nutrition status. In this paper, we suggest a simple way of incorporating unsynchronized short-term vertical migration of zooplankton into the mean-field modelling framework. We compute grazing of zooplankton in each layer depending on feeding activity of organisms in the layer. We take into account grazing impact of animals which are in the active phase of foraging cycle at the given moment of time but neglect the impact of animals which are in the non-active phase of the cycle (e.g. digesting food). Unsynchronized vertical migration determines the vertical distribution of actively feeding animals in layers depending on vertical distribution of food. In this paper, we compare two generic plankton models: (i) a model based on ‘classical’ grazing approach and (ii) a model incorporating food-mediated unsynchronized vertical migration of zooplankton. We show that including unsynchronized food-mediated migration would make the behaviour of a plankton model more realistic. This would imply a significant enhancement of ecosystem's stability and some additional mechanisms of regulation of algal blooms. In the system with food-mediated unsynchronized vertical migration, the control of phytoplankton by herbivorous becomes possible even for very large concentrations of nutrients in the water (formally, when the system's carrying capacity tends to infinity).     

The structure and diurnal variations of the zooplankton of the Prince Edward Islands: Implications for the biomass build-up of higher trophic levels

Summary Short-term variations in the micro-, meso- and macrozooplankton communities at shelf and offshore stations in the vicinity of the Price Edward Islands were examined during April/May 1985 and 1986. Microzooplankton was dominated by copepod nauplii. Other holoplanktonic groups were represented in large numbers, while meroplanktonic larvae of benthic invertebrates were very scarce despite the large benthic population of the island shelf. Copepods dominated in terms of numerical abundance the meso- and macrozooplankton assemblage but cuphausiids were also very important in terms of biomass contribution. Dirunal vertical migrations were most pronounced for the euphausiids and the copepod Metridia gerlachei. The copepods Eucalanus longipes, Rhincalanus gigas and Microsetella sp. showed significant, but lesser, diurnal vertical migrations. A daylight decrease in zooplankton biomass was observed consistently on the island shelf, compared to the pattern found in deep-water. Data from daytime tows collected during April/May 1983 show that zooplankton biomass in the area increases with depth up to a stability level of about 50 mg/m³ (dry weight). It is suggested that an interaction between vertical migrations, surface Ekman drift and bottom topography results in zooplankton mass depletion by visual predators during daytime and replenishment during the night. The nocturnal advection of allochtonous zooplankton into the area may represent an input of food supply equivalent to as much as 2.2 times the local maximum phytoplankton production.     

Bottom-up regulation of a pelagic community through spatial aggregations

The importance of spatial pattern in ecosystems has long been recognized. However, incorporating patchiness into our understanding of forces regulating ecosystems has proved challenging. We used a combination of continuously sampling moored sensors, complemented by shipboard sampling, to measure the temporal variation, abundance and vertical distribution of four trophic levels in Hawaii's near shore pelagic ecosystem. Using an analysis approach from trophic dynamics, we found that the frequency and intensity of spatial aggregations-rather than total biomass-in each step of a food chain involving phytoplankton, copepods, mesopelagic micronekton and spinner dolphins (Stenella longirostris) were the most significant predictors of variation in adjacent trophic levels. Patches of organisms had impacts disproportionate to the biomass of organisms within them. Our results are in accordance with resource limitation-mediated by patch dynamics-regulating structure at each trophic step in this ecosystem, as well as the foraging behaviour of the top predator. Because of their high degree of heterogeneity, ecosystem-level effects of patchiness such as this may be common in many pelagic marine systems.     

Biochemical composition of crustacean zooplankton and their grazing on phytoplankton and ciliated protozoans in a recently founded reservoir (Sahela, Morocco

In order to assess the impact of crustacean zooplankton on phytoplankton and protozoan ciliates in the Sahela reservoir under semi-arid climate, we conducted experiments during the period from July to December 1999 at the deepest point in the lake (15 m). Samplings and measurements were carried out in diffusion chambers submerged in situ over a period of 7 h without (control chambers) and with (experimental chambers) crustacean zooplankton. During these experiments, counts were conducted on phytoplankton and ciliates to determine the abundance and the mortality of these organisms due to zooplankton in each diffusion chambers at t = 0 and t = 7 h of incubation. The study showed that the growth rates of phytoplankton and ciliates populations varied between 0.02 and 0.05 h-1 and from 0.01 to 0.07 h-1, respectively. The mortality caused by zooplankton grazing fluctuated from 0.07 to 0.2 h-1 of phytoplankton and from 0.01 to 0.2 h-1 of ciliates. These mortalities were significantly and positively correlated with the growth rates (r = 0.8; p < 0.02; n = 9). Moreover, the heavy predation by the crustacean zooplankton was exerted on small-sized phytoplankton and ciliates and we demonstrated the relationships between protozoans and zooplankton for the transfer of matter and energy in aquatic food webs. Furthermore, the crustacean zooplankton metabolism was different, whether zooplankton was present in diffusion chambers or in the lake.     

On the Relations of Vertical Distribution,Diurnal Migration and Nutritional State of Herbivorous Zooplankton in the Northern North Sea during FLEX 1976

At a fixed station in the northern North Sea the occurrence of herbivorous and omnivorous zooplankton and the changes in its vertical distribution were studied over 2 1/2 months during the spring phytoplankton bloom in 1976. From a sporadical distribution of relative few plankton organisms in the mixed water column at the end of winter, in the middle of April a significant accumulation of organisms began to occur in the upper layers parallel to the development of stratification and the phytoplankton spring bloom. As exceptions, the populations of Microcalanus pusillus and Metridia lucens withdrew to below the thermocline. About ten days after the decline of the phytoplankton bloom, a few zooplankton species—especially wax ester producing copepods like Calanus finmarchicus, Pseudocalanus elongatus, Microcalanus pusillus, aad Metridia lucens—started diurnal vertical migrations. The movements of the species and stages occurred synchronously, leading in the same direction. Nevertheless, the amplitude of the migrations increased with each maturity stage reached. The vertical migrations, characterized by actual locations and velocities of the centers of gravity of the populations, are interpreted in terms of metabolism leading to fat storage, decomposition and reconstruction of lipids and proteins, in terms of a periodical change in the density of the organisms. Further data sets about the vertical distribution of organisms are cited to support the hypotheses stated.     

Zooplankton of the northern Benguela region in a quiescent upwelling period

The general features of the horizontal and vertical distributionof the main zooplankton groups in the northern Benguela upwellingregion during a period of abated upwelling in April 1986 arediscussed. Three different types of water were detected: (i)an inshore strip of recently upwelled water; (ii) oceanic waterover the slope; and (iii) Angola Current water penetrating fromthe north. Abundance of the most representative zooplanktongroups, as well as of the total volume of zooplankton, was highestin the shelf region, particularly south of Walvis Bay. Zooplanktonwas most abundant in areas where phytoplankton concentrationswere high and offshore transport was low. There were significantdifferences between the abundance levels of copepods, euphausiids,chaetognaths, and fish eggs and larvae in the different layersof the water column sampled, with the highest concentrationsin the surface layers. The sharpest vertical gradients wereobserved at stations affected by Angolan water, where the thermoclinewas very strong. In contrast, inshore, where stratificationwas low, no such vertical gradients existed.     

Zooplankton-phytoplankton interactions in a eutrophic lake

Enclosure experiments were made in a cyanobacteria dominatedlake (Lake Rotongaio) to assess the impact of zooplankton (>150µm) grazing on algal growth rates and determine the effectof diel and vertical changes in zooplankton grazing intensityand nutrient (NH₄-N) regeneration upon abundance of phytoplankton.The filamentous cyanobacterium Anabaena minutissima var. attenuataand diatom Cyclotella meneghiniana showed a negative linearchange in abundance with a gradient in zooplankton grazing intensity.Phytoflagellates were not grazed and showed a positive linearchange in abundance with increasing zooplankton biomass. Theseeffects, as well as shortening of filament length of Anabaena,were caused by raptorial feeding by the alanoid copepod Boeckellapropinqua which dominated the zooplankton. Phytoplankton growthwas not stimulated by addition of nutrients, suggesting nutrientregeneration was not important. Diel and vertical changes infeeding and NH₄-N regeneration rates were measured in Marchand June 1988. Diel differences were more pronounced in Marchwhen the water column was stratified. Specific feeding rateswere more important than vertical changes in zooplankton biomassin determining community grazing rates in March, but in Junewhen the water column was mixed, vertical distribution of zooplanktonbiomass was important. Zooplankton grazing was an importantloss process for phytoplankton in the lower part of the epilimnionin Lake Rotongaio.     

Strength of coupling between phyto- and zooplankton in Lake Lucerne (Switzerland) during phosphorus abatement subsequent to a weak eutrophication

The strength of coupling between phyto- and zooplankton was measured from 1961 to 1995 by comparing the grazing effect of zooplankton (visible as clear-water phase only in 1968-1994) and also by excluding zooplankton in limnocorral experiments (1980-1984). Although long-term (1961-1995) measurements show little evidence of temporal changes in total biomass of phytoplankton or zooplankton, there is strong evidence of changes in the strength of coupling due to top-down effects. The ratio of change in biomass caused by cladocerans in the intensive grazing period of each year (May/June) and the recovery of netplankton after this period seems to be strongly influenced by the trophic state of the lake. When Lake Lucerne was mesotrophic (1971-1982), the annual mean of monthly changes in phytoplankton biomass was in the range of 1-2, indicating that the biomass more than doubled (or halved) from month to month (no change = 0). Under oligotrophic conditions, the annual average of monthly changes in biomass was below 0.5. Grazing measurements in limnocorrals at 2 m depth with labelled food (Rhodomonas lacustris) showed distinct diel rhythms, with maximum community grazing rate at dusk and dawn. These diel changes were caused by vertical migration of the zooplankton. Grazing rate and zooplankton biomass were strongly coupled, with a maximum rate of 100-200 ml day^-1 mg^-1 (zooplankton biomass) when daphnids were dominant. The decrease in biomass caused by excessive grazing shows parallel trends in nanoplankton and netplankton. However, the increase in biomass after the clear-water phase was largely caused by netplankton.      

Control mechanisms of diel vertical migration: theoretical assumptions

We explore control mechanisms underlying the vertical migration of zooplankton in the water column under the predator-avoidance hypothesis. Two groups of assumptions in which the organisms are assumed to migrate vertically in order to minimize realized or effective predation pressure (type-I) and to minimize changes in realized or effective predation pressure (type-II), respectively, are investigated. Realized predation pressure is defined as the product of light intensity and relative predation abundance and the part of realized predation pressure that really affects organisms is termed as effective predation pressure. Although both types of assumptions can lead to the migration of zooplankton to avoid the mortality from predators, only the mechanisms based on type-II assumptions permit zooplankton to undergo a normal diel vertical migration (morning descent and evening ascent). The assumption of minimizing changes in realized predation pressure is based on consideration of DVM induction only by light intensity and predators. The assumption of minimizing changes in effective predation pressure takes into account, apart from light and predators also the effects of food and temperature. The latter assumption results in the same expression of migration velocity as the former one when both food and temperature are constant over water depth. A significant characteristic of the two type-II assumptions is that the relative change in light intensity plays a primary role in determining the migration velocity. The photoresponse is modified by other environmental variables: predation pressure, food and temperature. Both light and predation pressure are necessary for organisms to undertake DVM. We analyse the effect of each single variable. The modification of the phototaxis of migratory organisms depends on the vertical distribution of these variables.     

Zooplankton of the Patagonian slope and adjacent waters in the autumn

In autumn, from March until May 1988 in the Patagonian shelf and slope zones, the system of the water current was modified and water temperature decreased. The biomass of seston and abundance of zooplankton decreased simultaneously, especially those of the subantactric species Calanus simillimus. Zooplankton organisms started their descent to deeper waters. The Antarctic species Rhincalanus gigas was common in the upper layers in March. In May, R. gigas was rare and occurred in deeper layers. Subtropical zooplankton species dominated in the shelf zone, while subantartic species dominated in the slope zone within the Falkland current. In May, the abundance of shelf species in the slope zone increased due to weakening of the Falkland current and its dislocation to the east. The zooplankton biomass and abundance increased at the zone of water divergence in the east and at the slope frontal zone in the west.     

The role of highly unsaturated fatty acids in aquatic foodweb processes

1. Polyunsaturated fatty acids (PUFA) are almost exclusively synthesized by plants. Animals can convert from one form of PUFA to another through elongation and desaturation, but very few can synthesize PUFA de novo. PUFA play an important role in regulating cell membrane properties, serve as precursors for important animal hormones and are essential for animals. 2. In aquaculture studies, highly unsaturated fatty acids (HUFA), a subset of PUFA, have been found to be critical for maintaining high growth, survival and reproductive rates and high food conversion efficiencies for a wide variety of marine and freshwater organisms. 3. The plankton literature suggests high food-quality algae species are rich in HUFA and low food-quality algae are poor in HUFA. Adding semi-pure emulsions of HUFA to algae monocultures can markedly increase the growth rates of zooplankton feeding on these mixtures. 4. A study measuring zooplankton biomass accrual when feeding on natural phytoplankton found a strong correlation between phytoplankton HUFA (specifically eicosapentaenoic acid) content and herbivorous zooplankton production. 5. The aquatic ecology literature suggests that planktonic foodwebs with high HUFA content phytoplankton have high zooplankton to phytoplankton biomass ratios, while systems with low HUFA phytoplankton have low zooplankton biomass. Also, the seasonal succession of plankton in many temperate lakes follows patterns tied to phytoplankton HUFA content, with intense zooplankton grazing and ‘clear-water-phases’ characteristic of periods when the phytoplankton is dominated by HUFA-rich species. 6. Herbivorous zooplankton production is constrained by the zooplankton’s ability to ingest and digest phytoplankton. It is becoming increasingly clear, however, that much of the phytoplankton which is assimilated may be nutritionally inadequate. HUFA may be key nutritional constituents of zooplankton diets, and may determine energetic efficiency across the plant–animal interface, secondary production and the strength of trophic coupling in aquatic pelagic foodwebs.     

Mesozooplankton biomass and indices of grazing and metabolic activity in Antarctic waters

Mesozooplankton abundance, body area spectrum, biomass, gut fluorescence and electron transfer system (ETS) activity were studied in the Antarctic Peninsula during the post-bloom scenario in these waters. Values of abundance and biomass were rather low and decreased sharply from the slope waters to the coastal area. In contrast, specific gut fluorescence and ETS activity were high in the coastal area and decreased through the shelf-break. Large copepods were very scarce, similarly to the post-bloom conditions in phytoplankton where large cells are not abundant and small cells such as flagellates dominate the water column. The vertical distribution showed two well defined layers by day, one at the surface which corresponded to krill organisms and a second at depth (>300 m) formed mainly by the large copepod Metridia gerlachei. During the short night, this layer ascended at the time that krill at the surface migrated to deeper waters as observed from acoustics and net sampling. This observation and the absence of large copepods over the shelf suggest that krill consumption on large phytoplankton cells during the bloom is followed by an increase in predation upon mesozooplankton. It also suggests that krill decrease the abundance and biomass of mesozooplankton over the shelf and continues their predation upon mesopelagic copepods during the post-bloom in Antarctic waters. This behaviour could explain the long ago described impoverishment in mesozooplankton south of the Antarctic Circumpolar Current.     

Are zooplankton food resources poor in the vegetated littoral zone of shallow lakes?

1. The distribution of zooplankton in shallow lakes is negatively related to macrophyte density. However, the abundance of their food along density gradients of macrophytes is unknown. A common but untested assumption is that food quantity and quality for pelagic zooplankton is poor in the littoral zone owing to the deleterious influence of macrophytes on phytoplankton. 2. We tested this assumption with a combination of a field survey and laboratory experiments. We collected seston samples from the littoral and pelagic zones of four shallow temperate lakes and related food quantity (phytoplankton biovolume) and quality to macrophyte abundance (per cent volume infested). Seston food quality was assessed in three ways: N/C and P/C ratios, polyunsaturated fatty acid content and phytoplankton community composition. In the laboratory, we measured the growth and reproduction of Daphnia pulicaria on diets consisting of seston from the littoral and pelagic zones in one lake. 3. In our four study lakes, food quantity was not significantly influenced by macrophyte abundance, and food quality was generally high. Laboratory experiments showed increased juvenile growth, but no significant change in D. pulicaria reproduction, when feeding on littoral resources compared to pelagic resources. 4. Our results suggest that there is no nutritional cost to pelagic zooplankton inhabiting the littoral zone. Therefore, it is likely that other factors (e.g. predation, abiotic factors) are involved in determining zooplankton habitat use.     

Trophic structure in open waters of the marginal ice zone in the Scotia-Weddell confluence region during spring (1983)

The structure of the food web was investigated in open waters adjacent to the marginal ice zone in the southern Scotia Sea in spring 1983. Diets were defined for dominant zooplankton, micronekton, and flying seabird species and then aggregated by cluster analysis into feeding groups. Most zooplankton were omnivorous, feeding on phytoplankton, protozoans, and in some cases, small metazoans (copepods). Only two species were found to be exclusively herbivorous:Calanoides acutus andRhincalanus gigas. Micronekton were carnivores with copepods being the dominant prey in all their diets. The midwater fishElectrona antarctica was the dominant food item in seven of the nine seabird species examined. Cephalopods, midwater decapod shrimps and carrion were also important in the diets of a few seabird species. Comparison (cluster analysis) of diets in spring with other seasons (winter, fall) indicated that over half the species examined (18 of 31) had similar diets in all seasons tested. The significant intraspecific shifts in diet that did occur were attributable to regional, seasonal, and interannual effects. A scheme is presented that describes the major energetic pathways through the open water ecosystem from phytoplankton to apex predators. At the base are phytoplankton and protozoans which are the principal food resource for the biomass copepods and krill. Krill and the biomass copepods are the principal forage of the midwater fishElectrona antarctica which, in turn, is the central diet component of flying seabirds as well as important food for the Antarctic fur seal and cephalopods. Krill are a major diet element for the fur seal and cephalopods, and the principal food of the minke whale.     

Modelling the influence of copepod behaviour on faecal pellet export at high latitudes

The contribution of faecal pellet (FP) production by zooplankton to the downward flux of particulate organic carbon (POC) can vary from <1 % to more than 90 % of total POC. This results from varying degrees of interception and consumption, and hence recycling, of FPs by zooplankton in the upper mixed layers, and the active transport of FP to depth via diel vertical migration (VM) of zooplankton. During mid-summer at high latitudes, synchronised diel VM ceases, but individual zooplankton may continue to make forays into and out of the surface layers. This study considers the relative importance of different VM behaviours on FP export at high latitudes. We focussed on copepods and parameterised an individual-based model using empirical measures of phytoplankton vertical distribution and the rate of FP production, as a function of food availability. FP production was estimated under three different behaviours common to high-latitude environments (1) no VM, (2) foray-type behaviour and (3) synchronised diel VM. Simulations were also made of how each of these behaviours would be observed by an acoustic Doppler current profiler (ADCP). The model found that the type of copepod behaviour made a substantial difference to the level of FP export to depth. In the absence of VM, all FPs were produced above 50 m, where the probability of eventual export to depth was low. In foray-type scenarios, FP production occurred between 0 and 80 m, although the majority occurred between 30 and 70 m depth. Greatest FP production in the deeper layers (>70 m) occurred when diel VM took place. Simulated ADCP vertical velocity fields from the foray-type scenario resembled field observations, particularly with regard to the occurrence of positive anomalies in deeper waters and negative anomalies in shallower waters. The model illustrates that active vertical flux of zooplankton FP can occur at high latitudes even when no synchronised VM is taking place.     

Fine-scale spatial patterns in the coastal epiplankton off southern California

Fine-scale vertical (5 – 40 m) and horizontal (50 –500 m) patterns of temperature, chlorophyll and abundance ofzooplankton species were sampled with a pump filtration systemin the surface waters offshore of San Diego in May and October,1978. Intense and consistent patterns were most apparent invertical profiles. Herbivorous zooplankton were more consistentlyassociated with the estimated primary productivity maximum thanwith the deeper chlorophyll maximum layer, which representeda phytoplankton biomass maximum. Predators were positively correlatedwith abundant potential prey species. Variations in body lengthwith depth suggest that these fine-scale patterns were sufficientlystable to influence zooplankton growth. Consequences for grazingand predator – prey interactions in pelagic ecosystemsare discussed. ¹Present address: NOAA/NMFS Southwest Fisheries Center, PO Box271, La Jolla, CA 92038, USA     

Two types of maternal care for juveniles observed in Caprella monoceros Mayer, 1890 and Caprella decipiens Mayer, 1890 (Amphipoda: Caprellidae)

Spatial and seasonal patterns in phytoplankton and zooplankton communities of Lake St. Clair from June through September, 1984 are described. Phytoplankton biomass averages 586 µg l^-1 with the Diatomae and Chrysophyceae predominating. Zooplankton biomass averages 663 µg l^- with small bosminid Cladocera being the most abundant organisms. Lake St. Clair zooplankton biomass is second only to that of Lake Erie amongst the St. Lawrence Great Lakes. Biomass size spectra are typical in structure for mesotrophic lakes but low explained variance in the annual normalized spectrum is indicative of a perturbed system. Since 1972/1973 there appears to have been a slight decrease in zooplankton abundance in the lake accompanied by a shift from dominance of rotifers to dominance of cladocerans. We hypothesize that high flushing rate and seasonal variability coupled with contaminant loadings have resulted in a plankton community reduced in taxonomic diversity and dominated by small-bodied species.     

A Short-Term Study of Vertical and Horizontal Distribution of Zooplankton During Thermal Stratification in Lake Kinneret,Israel

This study documents for the first time both vertical and horizontal distribution patterns of the zooplankton community in Lake Kinneret during the period of thermal stratification. The zooplankton distribution patterns were explored in relation to abiotic (temperature, oxygen) and biotic (picocyanobacteria, ciliates, flagellates, phytoplankton, fish) environmental gradients. Sampling was carried out on 6–7 July 1992 at five stations and six depths from nearshore to offshore. Zooplankton abundance and biomass varied from 5 to 267 ind. l^–1(mean: 95 ind. l^–1), and from 0.1 to 65 d.w. mg m^–3(mean: 24 d.w. mg m^–3). Zooplankton taxonomic groups (Rotifera, Cladocera, Cyclopoida, Calanoida) and size classes (micro-, meso- and macrozooplankton) showed peaks of maximal density and biomass in the epilimnetic and metalimnetic strata (5 and 14 m). Depth, accounting for 31–39% of total spatial variation, reflected the vertical distribution of zooplankton in relation to temperature and oxygen declines, and the higher concentration of food resources (protists and phytoplankton) in the epilimnion and metalimnion. Onshore–offshore distance, accounting for 17–22% of the total spatial variance, reflected different distribution patterns shown among zooplankton groups and size classes. The macrozooplankton (Copepoda, Cladocera) was more abundant offshore, whereas microzooplankton (Rotifera and nauplii) predominated nearshore. These horizontal distribution patterns were related to small increases in temperature and phytoplankton biomass, and higher concentrations of fish in the littoral zone. Although limited to a short temporal scale, our study indicated that zooplankton spatial distribution in Lake Kinneret during the period of thermal stratification was related to physicochemical, food and predation factors, manifested differently along the vertical and nearshore–offshore gradients.     

Influence of hydrophyte abundance on the spatial distribution of zooplankton in selected lakes in Greece

Submerged hydrophyte vegetation consists of a highly important biotic component of maintaining lake ecosystems towards a “clear water” ecological status. Aquatic macrophytes are well known to play a significant multidimensional role in lakes by competing with phytoplankton growth, stabilising sediment and offering refuge to fishes, macro-invertebrates and littoral zooplankton, amongst others. Zooplanktons that are associated with macrophyte beds, in particular, may act as a positive feedback mechanism that contributes to maintaining a clear-water state. Although there are several studies investigating the relationships between macrophytes and zooplankton in European lakes, few have yet been carried out in Greek lakes. Seasonal field sampling was conducted from spring 2006 to autumn 2008 in four lakes of northwestern Greece. Zooplankton samples were collected from within hydrophyte beds in each lake to estimate their relative abundance and species density. Hydrophyte abundance and composition was recorded on a five-point scale. Moreover, water samples were analysed to determine nutrient and chlorophyll-a concentration. Pearson correlations between zooplankton density and key physicochemical variables were conducted to distinguish significant abiotic variables related with major zooplankton groups. Kruskal–Wallis non-parametric analysis was used to test for significant differences in zooplankton composition and environmental variables amongst the five hydrophyte abundance classes. In addition, Canonical correspondence analysis was used to distinguish possible correlations amongst the macrophyte and zooplankton species. Zooplankton density was significantly higher in dense macrophyte vegetation. Small-sized species (e.g. Rotifera) dominated the zooplankton community, indicating the eutrophic nature of the lakes. Large Cladocera were present in low abundance and were mostly littoral. The current research contributes to a better understanding of relationships between biotic groups in selected Greek lakes.     

Environmental influences on long-term variability in marine plankton

The major patterns of geographical and seasonal variability of the plankton of the north-east Atlantic and the North Sea are described to provide the background to a presentation of the dominant patterns of year-to-year fluctuations in the abundance of the plankton of the area for the period 1948 to 1984. A feature of the variability is a marked similarity both between species and between areas. The main pattern of year-to-year change has the form of a quasi linear downward trend in abundance with, superimposed on this, an element of variability with a periodicity of about three years. There is a complex relationship between the plankton and an estimate of changes in the frequency of westerly weather which can be interpreted in terms of influences acting over limited periods of the seasonal cycle coupled with persistence in the stocks of zooplankton. Relationships between year-to-year variations in the abundance of phytoplankton and zooplankton can be interpreted in terms of a response by the zooplankton to variations in food supply coupled with feed-back to the phytoplankton involving in situ nutrient regeneration.