Cladoceran Biomass Research Articles

RELATIONSHIP BETWEEN FISH AND THE NUMBER OF HORNS IN CERATIUM HIRUNDINELLA (DINOPHYCEAE): A FOOD-WEB-MEDIATED EFFECT ON ALGAL MORPHOLOGY?1

In a freshwater mesocosm experiment, we explored the potential for direct and indirect effects of roach (Rutilus rutilus) and Eurasian perch (Perca fluviatilis), two planktivorous fishes with different feeding behaviors, on the morphology of Ceratium hirundinella (O. F. Müll.) Dujard., a large dinoflagellate. Three morphs were detected: one with two hypothecal horns, one with a third rudimentary horn, and one with three well-developed horns. We observed a strong negative relationship between the presence of fish and the proportion of three-horned cells. The two fishes had strikingly similar effects on C. hirundinella morphology, despite their different capabilities to retain particles of the size of C. hirundinella. This finding suggests that the morphological variation in C. hirundinella was not related to selection by fish. Morphological variations in C. hirundinella could not be explained by fish-mediated variations in turbidity (i.e., light climate) or by predation pressure by the fish. In contrast, the proportion of three-horned cells was directly related to the biomass of filter-feeding cladocerans. This result was unexpected since cladocerans are not considered to consume C. hirundinella and they did not depress C. hirundinella numbers in our experiment. Without excluding other possible mechanisms, we suggest that the third horn might help these dinoflagellates avoid physical contact with the filtering apparatus of the cladocerans and the consequent potential damage caused by these herbivores, which were more abundant in the absence of planktivorous fish.

Cool winters versus mild winters: effects on spring plankton in Lake Peipsi

The influence of two winter periods with a different duration of the ice cover on Lake Peipsi (Estonia) on plankton and nutrient content was analysed. The winters of 2005 and 2006 were cold with ice duration of 140 days, whereas the winters of 2007 and 2008 were mild with about 50 and 15 days of ice duration, respectively. Total phosphorus (TP) concentration was lower, while silicon content and the nitrogen-phosphorus ratio (TN : TP) were markedly higher in the springs after the short winters of 2007 and 2008. The high Si concentration and TN : TP ratio persisted throughout the growing season of those years. Unicellular centric diatoms showed a sharp increase in April 2008, while the large filiform diatom Aulacoseira islandica dominated in the cool winters and after ice break-up. The high spring peak of diatoms was followed by their low biomass in summer. In May 2008, total zooplankton biomass, cladoceran biomass, and rotifer biomass were two times and that of copepods three times as high as in the Mays after the long-lasting ice cover. In the Junes after the mild winters the biomasses of total zooplankton and both crustacean groups (Cladocera, Copepoda) were about two times as high as the corresponding indicators for the Junes after the cool winters. The biomass of rotifers, on the contrary, was two times lower in the Junes after the warm winters because the numerous cold stenotherms Polyarthra dolichoptera and Synchaeta verrucosa had totally disappeared from zooplankton. The influence of ice duration on phyto- plankton is most likely indirect, acting through nutrients, and on zooplankton direct, acting through water temperatures. The springs after warm winters related positively to zooplankters' mean weight, zooplankton-phytoplankton biomass ratio, and the timing of the clear water period.

Seasonal succession and composition of the zooplankton community along an eutrophication and salinity gradient exemplified by Danish waters

The seasonal distribution of the major components of the zooplankton community, protozooplankton, copepods and cladocerans, along a eutrophication gradient were examined in order to establish if eutrophication through increases in phytoplankton biomass and productivity has an impact on biomass and composition of the zooplankton community. Data on salinity, temperature, inorganic nutrients, chlorophyll a, phytoplankton biomass and production, protozooplankton and mesozoo-plankton biomass were collected during routine sampling at seven stations representing a eutrophication gradient from open to estuarine waters. Mesozooplankton biomass varied seasonally in all areas with a unimodal pattern characterized by a peak between mid-spring and mid-summer. Total mesozooplankton biomass as well as copepod and cladoceran biomass, decreased with DIN concentration. Salinity and the type of ecoystem (well-mixed shallow estuaries versus stratified open waters) were the major factors controlling the biomass proportion of copepod species. Annual production of the total copepod community as well as the total grazing impact of copepods on primary production was higher in open waters than in estuarine waters. In estuarine type ecosystems, phytoplankton production is underexploited by copepod grazing whereas in the open type ecosystems, the phytoplankton production alone could not satisfy the carbon demand of copepods stressing the potential importance of protozoans in the copepod diet.

Effect of winter conditions on spring nutrient concentrations and plankton in a large shallow Lake Peipsi (Estonia/Russia)

During the study period 1997–2007, the duration of the ice cover of Lake Peipsi, the largest freshwater lake in Estonia, was variable. The ice-break took place in mid-April instead of generally in May. Compared with data from 1960s onwards, a trend of shortening of the duration of ice-cover period is evident. In under-ice phytoplankton, diatoms, cryptophytes and flagellated chlorophytes predominated from March to June. In zooplankton, while thermophobic rotifers dominated in densities, copepods were dominant in biomass. The monthly sum of positive water temperatures correlated with the zooplankton in the following month and duration of the ice cover correlated with the biomass of all different zooplankton groups in May and June. The sum of water temperatures in April, May and June was positively correlated with the biomass of cladocerans and copepods, and negatively with the biomass of winter rotifers. Mild winters affected nitrogen and silica concentration positively and phosphorus concentration negatively.

Plankton development in Lake Jonsvatn, Norway, after introduction of Mysis relicta: a long-term study

Quantitative sampling of phyto- and zooplankton has been carried out for 25 yr in Lake Jonsvatn, Norway, in order to assess changes after the introduction of Mysis relicta. In Lille Jonsvatn, the outlet basin, the cladoceran populations collapsed 7 yr after the mysid introduction. The mean cladoceran biomass was reduced by 95 to 99% for a decade before Bosmina longispina, Holopedium gibberum and Daphnia galeata developed new populations. B. longispina and H. gib- berum almost disappeared again after a few years, while D. galeata was replaced by D. longispina, which unexpectedly maintained a relatively dense population in the presence of high mysid abun- dance. Rotifers declined significantly in Lille Jonsvatn after the mysid establishment, whereas cope- pod biomass did not change. Phytoplankton biomass increased during the first years after the clado- ceran collapse, but declined later to levels lower than in pre-mysid years. The changes were probably a combined effect of reduced nutrient loads and grazing pressure. In the main basin, Store Jonsvatn, there were no detectable effects on the zooplankton community during the first 10 to 15 yr after the introduction, in spite of development to common densities of mysids within 8 yr. Over time, cladocerans decreased significantly, however, and mean biomass for the last 5 yr showed a 60% reduction compared with the first 10 yr. No significant long-term changes were detected in copepods, rotifers or phytoplankton biomass in Store Jonsvatn. Differences in tem- perature, stratification, light transmission and depth may partly explain the different plankton development in the 2 basins.

Recent shifts in the crustacean zooplankton community of Lake Huron

Cladoceran populations in the open waters of Lake Huron declined abruptly in 2003 and have since remained at historically low levels. The two dominant cladocerans, Daphnia mendotae and Bosmina longirostris , have been nearly extirpated from the northern region of the lake and are present in only slightly greater numbers in the south. Average nonpredatory cladoceran biomass in the lake has declined over 90% between 1998–2002 and 2003–2006. In addition, historically unprecedented declines in cyclopoid copepods were seen in the lake in 2005. These changes have occurred against the backdrop of declining nutrient levels in the lake and have coincided closely with declines in the amphipod Diporeia . We speculate that a combination of reduced primary production in the open waters and intensified planktivory due to the continuing disappearance of Diporeia has accounted for the losses in crustacean biomass seen in recent years.

Seasonal Variations in the Abundance and Biomass of Microcrustaceans in Relation to Environmental Variables in Two Shallow Tropical Lakes within the Cross River Floodplain, Nigeria

The diversity, abundance and biomass of microcrustaceans were studied monthly in Lakes Ehoma and Iyieke from March 2005 to August 2006. Twenty-six (26) species were identified and Daphnia obtusa was recorded for the first time in Nigerian freshwater. Contrary to our expectations, diversity, total biomass and cladoceran biomass were significantly higher during the rainy season (p < 0.05), and inter-lake variation in these was not significant, although the abundance of copepods was significantly higher at Ehoma. Diaphanosoma excisum accounted for over 50% of the total zooplankton biomass in the two lakes, low total biomass during the dry season was mainly due to the low population of this species. A well-defined succession pattern in which the cladocerans Moina micrura and D. excisum were predominant during pre-rainy and rainy seasons and the copepods Thermocyclops neglectus and T. decipiens dominant during dry and post-dry seasons was discerned. Canonical correspon dence analysis showed that zooplankton succession and biomass were influenced by seasonal variations in the major environmental variables (temperature, dissolved oxygen, lake morphometry and pH) of the lakes. These variables were in turn influenced by the seasonal flooding of the lakes. Since the attainment of maximum zooplankton biomass coincided with the breeding season of the major fishes of the Cross River, we therefore propose that factors such as climatic changes and the damming of the Cross River that could alter the flooding pattern of the lakes may inadvertently affect the zooplankton community structure of the lakes and consequently the fish productivity of the entire Cross River ecosystem.

Long-term zooplankton body size and species changes in a subtropical lake: implications for lake management

Analysis of long-term (1970-2002) zooplankton body sizes in Lake Kinneret, as recorded in an histori- cal zooplankton sample archive, combined with re-analysis of published long-term zooplankton taxonomic abun- dance data revealed previously undetected patterns in zooplankton biomass and demonstrates a profound long-term impact of planktivorous fi sh predation intensity driven by fiharvest practices. During the 1970s, crustacean zooplankton biomass remained relatively constant at a mean (±SD) of 1.50 ± 0.26 g m -3 , dropped precipitously in the 1980s reaching 0.57 g m -3 by 1993, and rebounded to 1.33 ± 0.19 g m -3 in the early 2000s. The 1980s decline refl ects decreased cladoceran biomass due to a decline in mean body mass in the three dominant cladoceran genera (Diaphanosoma, Ceriodaphnia, Bosmina) and decreased copepod biomass that resulted from a shift in dominance by large Mesocyclops ogunnus to small Thermocyclops dybowskii. Although crustacean body sizes remained small through the early 2000s, cladoceran and copepod densities increased by ~34 and > 300 %, respectively, after 1994 leading to a dramatic increase in crustacean zooplankton biomass. These patterns indicate an intensifi cation in fi predation throughout the 1980s and 1990s, and although zooplankton biomass rebounded in 1996-2002 to ~80 % of the 1970s levels due to sharp increases in crustacean densities, smaller cladoceran and copepod body sizes sug- gest that Lake Kinneret zooplankton continue to be strongly infl uenced by intense fi sh predation.

Influence of plankton mercury dynamics and trophic pathways on mercury concentrations of top predator fish of a mining-impacted reservoir

Physical and biogeochemical characteristics of the aquatic environment that affect growth dynamics of phytoplankton and the zooplankton communities that depend on them may also affect uptake of methylmercury (MeHg) into the pelagic food web of oligotrophic reservoirs. We evaluated changes in the quality and quantity of suspended particulate material, zooplankton taxonomy, and MeHg concentrations coincident with seasonal changes in water storage of a mining-impacted reservoir in northern California, USA. MeHg concentrations in bulk zooplankton increased from 4 ng·g–1at low water to 77 ± 6.1 ng·g–1at high water and were positively correlated with cladoceran biomass (r = 0.66) and negatively correlated with rotifer biomass (r = –0.65). Stable isotope analysis revealed overall higher MeHg concentrations in the pelagic-based food web relative to the benthic-based food web. Statistically similar patterns of trophic enrichment of MeHg (slopes) for the pelagic and benthic food webs and slightly higher MeHg concentrations in zooplankton than in benthic invertebrates suggest that the difference in MeHg bioaccumulation among trophic pathways is set at the base of the food webs. These results suggest an important role for plankton dynamics in driving the MeHg content of zooplankton and ultimately MeHg bioaccumulation in top predators in pelagic-based food webs.

Seasonal fluctuations in macrophyte cover and water transparency of four brown-water lakes: implications for crustacean zooplankton in littoral and pelagic habitats

The dynamics of crustacean zooplankton in the littoral and pelagic zones of four forest lakes having variable water qualities (colour range 130–340 mg Pt l−1, Secchi depth 70–160 cm) were studied. The biomass of zooplankton was higher in the littoral zone than in the pelagic zone only in the lake having the highest transparency. In the three other lakes, biomass was significantly higher in the pelagic zone than in the littoral zone. In the two lakes with highest transparency, the littoral biomass of cladocerans significantly followed the development of macrophyte vegetation, and cladoceran biomass reached the maximum value at the time of highest macrophyte coverage. In lakes with lowest transparency, littoral zooplankton biomass developed independently of macrophyte density and decreased when macrophyte beds were densest. The seasonal development of the littoral copepod biomass did not follow the development of macrophytes in any of the lakes. The mean size of cladocerans in the pelagic zone decreased with increasing Secchi depth of the lake, whereas in the littoral zone no such phenomenon was detected. Seasonally, when water transparency increased temporarily in two of the lakes, the mean size of cladocerans in the pelagic zone decreased steeply. For copepods, no relationship between water transparency and body size was observed. The results suggested that in humic lakes the importance of the littoral zone as a refuge decreases with decreasing transparency of the water and that low water transparency protects cladocerans from fish predation. All the observed between-lake differences could not be explained by fish predation, but were probably attributed to the presence of chaoborid larvae with variable densities. Feeding efficiency of chaoborids is not affected by visibility and thus they can obscure the relationship between water quality, fish density, and the structure of crustacean zooplankton assemblages.

The Water Framework Directive: Setting the phosphorus loading target for a deep lake in Denmark using the 1D lake ecosystem model DYRESM–CAEDYM

We used the one-dimensional DYRESM–CAEDYM model to elucidate and quantify the influence of the external phosphorus loading on ecosystem dynamics in moderately deep Lake Ravn, which is situated in an agricultural landscape in Denmark. Model simulations were used to quantify the extent to which the external phosphorus loading must be reduced to meet upcoming lake ecological quality requirements according to the European Union Water Framework Directive (WFD). The model generally showed good agreement with observed data for temperature and oxygen from the epilimnion and hypolimnion during the calibration period (7 years) as well as the validation period (5 years); although peaks of oxygen concentrations in epilimnion during late spring often were underestimated. Phosphate and total phosphorus (TP) concentrations were generally well reproduced in both the epilimnion and hypolimnion, though hypolimnetic phosphorus was occasionally underestimated in late summer. There was also good agreement between monitored data and modelled biomass of diatoms and dinoflagellates as well as the zooplankton biomass of cladocerans and calanoid copepods, although the timing of biomass peaks occasionally deviated from observations. Root-mean-square-errors (RMSE), used to quantify the model error, were overall similar for the calibration and the validation period. Simulations of scenarios with a reduced external TP loading suggest that a substantial reduction (40–50%) of the TP loading is required if phytoplankton biomass is to drop to a level sufficiently low to meet the proposed WFD requirements (summer average <6.5 μg chlorophyll a l −1). The predicted outcomes of considerable loading reductions should, however, be treated with some caution, as the conceptual model in this study could not fully account for the changes in trophic structure occurring at radically reduced TP loading. This particularly applies to changes in the fish stock, which may have extensive cascading effects via increased zooplankton grazing on the phytoplankton when external TP loading is reduced. This would most likely lead to higher transparency than that predicted by the model. To further improve the reliability of the predicted outcomes of various model scenarios, future work should include a series of test simulations also including fish predation. Today, fish predation is available in the DYRESM–CAEDYM model as well as other dynamic lake models; however, only sparse data is available on fish stock dynamics in Lake Ravn (as for most other lakes). An applicable test of the fish algorithms in today’s dynamic models will require reliable estimates of fish stock biomass in the study lakes during both the model calibration and validation period.

Limnological and ecological characteristics of tropical highland reservoirs in Tigray, Northern Ethiopia

The semi-arid highlands of Northern Ethiopia (Tigray) have numerous small reservoirs that have been created by microdams in an attempt to provide water supply for irrigation and livestock drinking. Although the reservoirs have substantial added value to residents, their use as water resource is jeopardized by eutrophication and a high occurrence of blooms of toxic cyanobacteria. So far, there is no systematic information available on the limnological and aquatic ecological characteristics of these dams. We carried out a standardized survey of 32 reservoirs and assessed a wide set of morphometric, abiotic and biotic variables. The sampling was performed during two seasons, September–October 2004 (end of the wet season/start of the dry season) and April–May 2005 (towards the end of the dry season). Using multivariate analysis, we revealed dominating patterns of variable associations and compared the variability in these patterns among seasons. According to standardized PCA and RDA analyses, the most important axis of variation was mainly represented by a gradient in nutrients and altitude that was also positively associated with phytoplankton biomass, suspended matter and oxygen concentration, and negatively with water transparency. For most variables, correlations between the wet and dry season were weak, which suggests that individual reservoirs behaved rather differently in their response to seasonal changes. Nevertheless, a Mantel correlation (r = 0.32; P = 0.035) showed a weak but significant overall concordance in the variable association patterns among seasons. A number of reservoirs became very shallow or fell dry in the dry season, a process that was associated with an increase in suspended matter and conductivity and a decrease in transparency. These reservoirs contained lower amounts of fish and tended to be less eutrophic than the deeper, permanent systems, as they had lower levels of phosphorus and chlorophyll-a. With multiple regression analysis, we constructed most parsimonious models in an attempt to explain the variation in key biotic variables: phytoplankton and cyanobacteria biomass, cladoceran biomass, fish biomass and the abundance of submerged vegetation. Phytoplankton and fish biomass tended to be positively related with the concentration of total phosphorus, whereas cladoceran biomass was not associated with nutrient concentrations. The positive association of fish and phytoplankton with nutrient concentrations suggests a bottom-up control, whereas the absence of an association between zooplankton and nutrient concentrations may be indicative for top-down control. The biomass of cyanobacteria was negatively related to the biomass of cladocerans (Daphnia), which likely reflects a top-down effect. Most reservoirs were turbid. The occurrence and abundance of macrophytes tended to be positively related to water transparency and was negatively associated to TP and the amount of livestock frequenting the reservoirs. However, macrophytes were not limited to clear-water reservoirs.

Changes in the Crustacean Communities of the Central Basin of Lake Erie during the First Full Year of the Bythotrephes longimanus Invasion

The exotic predatory cladoceran Bythotrephes longimanus was first observed in Lake Erie on 19 September, 1985. During the early summer immediately prior to its appearance, the cladoceran community in the central basin of Lake Erie was characterized by large populations of Bosmina longirostris, Eubosmina coregoni, Daphnia mendotae, and Daphnia retrocurva, with the latter three species persisting throughout August and September, along with Diaphanosoma spp. Community composition during early summer 1986 was similar to that of the previous year, but densities of all cladocerans decreased dramatically coincident with the appearance of Bythotrephes in mid-July, and remained suppressed throughout August and September. Only D. mendotae was present in appreciable numbers during this period; from mid-July through the end of August, 86-98% of cladoceran biomass (exclusive of Bythotrephes and Leptodora) was contributed by D. mendotae. Densities of Leptodora and Bythotrephes showed a strikingly inverse relationship, both temporally and spatially, during 1986. Size frequency distributions of D. mendotae exhibited an immediate shift towards extremely large ( > 2.5 mm) individuals coincident with Bythotrephes’ appearance in 1986, suggesting an upper size limit to efficient prey utilization by Bythotrephes. These results suggest that Bythotrephes can have substantial impacts both on cladoceran community composition, and on the size distributions of individual species.

A comparison of zooplankton densities and biomass in Lakes Peipsi and Võrtsjärv (Estonia): rotifers and crustaceans versus ciliates

The abundance and biomass of ciliates, rotifers, cladocerans and copepods were studied in Lake Peipsi and Lake Vortsjarv, both of which are shallow, turbid and large. Our hypothesis was that in a large shallow eutrophic lake, the ciliates could be the most important zooplankton group. The mean metazooplankton biomass was higher in Peipsi than in Vortsjarv (mean values and SD, 1.8 ± 0.7 and 1.3 ± 0.6 mg WM l−1). In Peipsi, the metazooplankton biomass was dominated by filtrators that feed on large-sized phytoplankton and are characteristic of oligo-mesotrophic waters. In Vortsjarv, the metazooplankton was dominated by species characteristic of eutrophic waters. The planktonic ciliates in both lakes were dominated by oligotrichs. The biomass of ciliates was much greater in Vortsjarv (mean 2.3 ± 1.4 mg WM l−1) than in Peipsi (0.1 ± 0.08 mg WM l−1). Ciliates formed about 60% of the total zooplankton biomass in Vortsjarv but only 6% in Peipsi. Thus, the food chains in the two lakes differ: a grazing food chain in Peipsi and a detrital food-chain in Vortsjarv. Consequently, top-down control of phytoplankton can be assumed to be much more important in Peipsi than in Vortsjarv. When the detrital food chain prevails, the planktonic ciliates become the most important zooplankton group in shallow, eutrophic and large lake. Neglecting protozooplankton can result in serious underestimates of total zooplankton biomass since two-thirds of the zooplankton biomass in Vortsjarv comprises ciliates.

Impact of the activities of beaver on the zooplankton of a piedmont river (Mongolia)

The activities of beavers, even in the absence of construction, promote the increase of abundance and the change of community composition in zooplankton. Size of populations, biomass, abundance of copepods, number of species, and value of the Shannon biodiversity indices all increase in parts of the river with some water flow. Biomass and abundance of cladocerans increase in stagnant floodplain waterbodies, along with the reduction of the number of species and of the uniformity of communities.

Bottom-up control of whitefish populations in ultra-oligotrophic Lake Brienz

Lake Brienz, an oligotrophic pre-alpine Swiss lake, went through a mesotrophic period between around 1960 and 1990. The lake is moderately turbid caused by fine suspended solids from glaciers. In 1999, yield of the economically important whitefish collapsed to about 10% of preceding years. Age and growth analysis of the two whitefish types examined – small and large type – revealed an almost complete halt of growth from 1999 until June 2000, paralleled by poor condition. Zooplankton data showed that cladocerans, the preferred food of whitefish, were rare from January 1999 until June 2000. In order to elucidate the trophic relationships between zooplankton and fish, the «Wisconsin Bioenergetics Model 3.0» was applied. The analysis showed that poor growth and condition of whitefish in 1999 and 2000 were caused by the scarcity of primary food organisms. The relatively small and slender fish could not be caught by legal gillnets, which resulted in poor fishing yield. Evidence is presented that cladoceran biomass governs food consumption by the fish («bottom- up effect»), while the effect of fish predation on cladocerans was found to be negligible, most likely also during the period of poor growth. Turbidity did not appear to significantly interfere with the feeding of whitefish. Growth, condition and commercial yield of whitefish partly increased again after 2000, but due to the very low productivity of Lake Brienz, fishing yield will remain low. Food chains in such oligotrophic systems are fragile. It is likely that a future collapse of the cladoceran population and, thus, the whitefish fishery will happen again.

Zooplankton biomass in tropical reservoirs in southern Brazil

In order to test the hypothesis that zooplankton biomass distribution (total and taxonomic groups) was influenced by the nutrient concentration and primary productivity distribution in three tropical reservoirs, subsurface samples were taken in the fluvial, transitional and lacustrine regions of three reservoirs (oligotrophic, mesotrophic and eutrophic) in southern Brazil (Parana State) in March and September 2002. Zooplankton biomass ranged from 0.04 to 264.47 mg DW m−3. Higher biomass values were observed for cladocerans (73.60%; 0.01–259.86 mg DW m−3), followed by copepods (22.05%; 0.01–69.69 mg DW m−3) and rotifers (4.35%; 0.01–11.52 mg DW m−3). In general, the total zooplankton, rotifer, cladoceran and copepod biomass, and chlorophyll-a and total nutrient concentrations showed a similar longitudinal distribution within the reservoirs. Total zooplankton, rotifer and cladoceran biomass were related to the chlorophyll-a concentration, and zooplankton biomass was related to the total phosphorus distribution. This may have been due to the significant multicolinearity between the chlorophyll-a and total phosphorus concentrations. Cyanobacteria influenced the taxonomic group biomass results by interfering with the filter feeding in larger zooplankton species, which favoured the dominance of smaller species. As regards the longitudinal distribution of copepod biomass, cyanobacteria biomass determined the displacement of the microcrustaceans to the fluvial region of Irai Reservoir. Our results supported the hypothesis formulated and the primary productivity was the major predictor of the zooplankton biomass distribution in the reservoirs.

Zooplankton biomass of reservoirs in different trophic conditions in the State of São Paulo, Brazil

This paper reports on a study involving an estimate of the biomass of rotifers, cladocerans, and cyclopoid and calanoid copepods found in reservoirs in different trophic conditions, comparing and relating numerical density data and pointing out differences between the dry and rainy seasons. In terms of numerical densities, both reservoirs were dominated by rotifers, although cladocerans represented a higher biomass in the oligotrophic environment in both seasons. In the eutrophic environment, higher biomass values were found for cyclopoids during the dry season and for cladocerans during the rainy one. Different biomass patterns were observed relating to both the trophic conditions and the rainy and dry seasons.

Response of fish and plankton to nutrient loading reduction in eight shallow Danish lakes with special emphasis on seasonal dynamics

Summary1. For 13 years the response of the plankton and fish community to a decline in external phosphorus loading was studied in eight lakes with a mean depth &lt;5 m. We conducted chi‐square analyses of sign of slope (positive or negative) of bimonthly averages of plankton variables for the eight lakes versus time. For fish, we compared results from two periods, i.e. 1989–1994 versus 1994–2001 as less data were available.2. Fish community structure tended to respond to the lowered concentration of total phosphorus (TP), although not all changes were significant. While catch per unit effort (multi‐mesh sized gill nets) of cyprinids (especially bream, Abramis brama and roach, Rutilus rutilus) was highest in the first 5‐year period, the quantitative importance particularly of perch (Perca fluviatilis), pike (Esox lucius) and rudd (Scardinius erythropthalmus), a littoral species, increased significantly after 1994.3. No changes occurred in zooplankton biomass, except for an increase in November and December. Biomass of small cladocerans, however, declined during summer and autumn, and the proportion of Daphnia to cladoceran biomass also increased. Average body weight of Daphnia and that of all cladocerans increased. The proportion of calanoids among copepods decreased in summer and the average body weight of cyclopoids and calanoids decreased during summer and autumn/early winter.4. Total biovolume of phytoplankton declined significantly in March to June and tended to decline in November and December as well, while no significant changes were observed during summer and autumn. Non‐heterocystous cyanobacteria showed a decreasing trend during summer and autumn, while heterocystous cyanobacteria increased significantly in late summer. An increase in late summer was also evident for cryptophytes and chrysophytes, while diatoms tended to decline during most seasons.5. We conclude that phytoplankton, and probably also fish, responded rapidly to reduced loading, whereas the effect on zooplankton was less pronounced. However, increases in body weight of cladocerans and the zooplankton to phytoplankton biomass ratio during summer indicate reduced top‐down control on zooplankton and enhanced grazing on phytoplankton. This conclusion is supported by a tendency for fish biomass to decline and a shift towards greater dominance by piscivores and, thus, an increased likelihood of predator control of zooplanktivorous cyprinids.

ZOOPLANKTON COMMUNITY STRUCTURE AFFECTS HOW PHYTOPLANKTON RESPOND TO NUTRIENT PULSES

Ecological theory suggests that community structure determines how eco- systems respond to perturbation. However, this idea is not well-tested empirically, in part because it is difficult to define the appropriate reference state for evaluating post-pertur- bation dynamics in field experiments. In this study, we (1) evaluated two different methods of determining the reference state, and (2) tested hypotheses derived from theory and previous experiments about the effects of herbivore community structure on autotroph responses to nutrient pulses in freshwater plankton communities. We hypothesized that (1) phytoplankton are less sensitive to a nutrient pulse with increasing zooplankton biomass, size, and Daphnia abundance, and (2) phytoplankton recover more quickly from the nutrient pulse with increased zooplankton biomass and increased abundance of taxa with rapid numerical responses, especially small-bodied cladocerans. We tested these hypotheses in field mesocosm experiments, which crossed four zooplankton community structures with two nutrient pulse treatments in a fully randomized factorial design. We found that phy- toplankton sensitivity decreased with increased zooplankton biomass, size, and Daphnia biomass regardless of the method used to define the reference state, supporting our first hypothesis and suggesting that results for sensitivity are robust to methodology. This result, together with previous work, clearly demonstrates that zooplankton have a strong effect on phytoplankton sensitivity. Further, as predicted, return times were shorter with increased zooplankton biomass and biomass of small cladocerans. However, contrary to expectations, return times also declined with Daphnia and copepod biomass, suggesting that the effects of zooplankton on return time are more complex than we hypothesized. Moreover, con- clusions for return time depended somewhat on methodology, indicating that we may need to reevaluate whether the concept of return time is relevant for field experiments.