Bjørn A. Faafeng

The impact of nutrient state and lake depth on top-down control in the pelagic zone of lakes: A study of 466 lakes from the temperate zone to the Arctic

Using empirical data from 466 temperate to arctic lakes covering a total phosphorus (TP) gradient of 2–1036 gL –1 , we describe how the relative contributions of resource supply, and predator control change along a nutrient gradient. We argue that (a) predator control on large-bodied zooplankton is unimodally related to TP and is highest in the most nutrient-rich and nutrient-poor lakes and generally higher in shallow than deep lakes, (b) the cascading effect of changes in predator control on phytoplankton decreases with increasing TP, and (c) these general patterns occur with significant variations—that is, the predation pressure can be low or high at all nutrient levels. A quantile regression revealed that the median share of the predatorsensitive Daphnia to the total cladoceran biomass was significantly related unimodally to TP, while the 10% and 90% percentiles approached 0 and 100%, respectively, at all TP levels. Moreover, deep lakes (more than 6 m) had a higher percentage of

Long-term responses to fish-stock reduction in small shallow lakes: interpretation of five-year results of four biomanipulation cases in The Netherlands and Denmark

The effects of fish stock reduction have been studies in 3 Dutch lakes (Lake Zwemlust, Lake Bleiswijkse Zoom and Lake Noorddiep) and 1 Danish lake (Lake Væng) during 4–5 years. A general response id described. The fish stock reduction led in general to a low fish stock, low chlorophyii-a, high transparency and high abunuance of macrophytes. Large Daphnia became abundant, but their density decreased, due to food limitation and predation by fish. The total nitrogen concentration became low due to N-uptake by macrophytes and enhanced denitrification. In Lake Bleiswijkse Zoom the water transparency deteriorated and the clear water state was not stable. The fish stock increased and the production of young fish in summer was high. lear water occurred only in spring. Large daphnids were absent in summer and the macrophytes decreased.In Lake Zwemlust, Lake Væng and Lake Noorddiep the water remained clear during the first five years. In summer of the sixth year (1992) transparency decreased in Lake Zwemlust (with high P-concentration of 1.0 mg P l-1). Also in Lake Væng (with a low nutrient concentration of 0.15 mg P.⋆-1) a short term turbid stage (1.5 month) occurred in summer 1992 after a sudden collapse of the macrophytes. Deterioration of the water quality seems to start in summer and seems related to a collapse in macrophytes. At a low planktivorous fishstock (e.g. Lake Væng)thhe duration of the turbid state is shorter. than in presence of a high planktivorous fish biomass (e.g. Lake Zwemlust, and later years of Lake Bleiswijkse Zoom).

Habitat shift in roach (Rutilus rutilus) induced by pikeperch (Stizostedion lucioperca) introduction: predation risk versus pelagic behaviour

Changes in the fish community structure and habitat use were followed after the introduction of pikeperch (Stizostedion lucioperca) to the roach-dominated Lake Gjersjøen. Quantitative echosounding showed that the density of juvenile roach (Rutilus rutilus) was dramatically reduced in pelagic areas, from 12 000–15 000 fish/ha to 250 fish/ha, while total fish density remained unchanged in littoral areas. At the same time, the habitat segregation between different size groups of roach was altered as larger roach utilized the pelagic zone after pikeperch introduction. The loss of the pelagic refuge for juvenile roach increased the availability of juvenile roach to littoral predators, notably perch. In littoral areas, the fish community changed from one dominated by roach (> 95%) to one dominated by perch (> 50%).

Extrinsic and intrinsic controls of zooplankton diversity in lakes

Pelagic crustacean zooplankton were collected from 336 Norwegian lakes covering a wide range of latitude, altitude, lake area, mean depth, production (as chlorophyll a), and fish community structure. Mean zooplankton species richness during the ice-free season was generally low at high latitudes and altitudes. Further, lower species richness was recorded in western lakes, possibly reflecting constraints on migration and dispersal. However, despite obvious spatial limitations, geographic boundaries were only weak predictors of mean zooplankton richness. Similarly, lake surface area did not contribute positively to mean richness such as seen in other ecosystem surveys. Rather, intrinsic factors such as primary production and fish community (planktivore) structure were identified by regression analysis as the major predictors of zooplankton diversity, while a positive correlation was observed between species richness and total zooplankton biomass. However, in spite of a large number of variables included in this study, the predictive power of multiple regression models was modest (<50% variance explained), pointing to a major role for within-lake properties, as yet unidentified intrinsic forces, stochasticity, or dispersal as constraints on zooplankton diversity in these lakes.

Competition or niche segregation between Holopedium and Daphnia; empirical light on abiotic key parameters

A survey of 346 Norwegian lakes revealed some consistent patterns with regard to the occurrence of Holopedium gibberum versus Daphnia spp. Both biomass and frequency of Holopedium were negatively correlated with pH and Ca-concentration, and this species rarely occurred at pH > 8, or Ca-concentrations > 10 mg l−1. On the contrary, the various Daphnia species, although showing different susceptibilities to fish predation and different preferences for lake productivity, were all positively correlated with Ca. Statistical analysis as well as laboratory experiments and biochemical considerations, suggest that Holopedium may be a superior competitor at very low Ca-concentrations, but is replaced by Daphnia at higher pH and increasing Ca-content. High pH in itself is possibly disadvantageous for Holopedium by preventing formation of the muco-polysaccaride mantle.

In situ measurement of algal growth potential in aquatic ecosystems by immobilized algae

Cells of the green algaSelenastrum capricornutum were immobilized in alginate beads. The alga was able to grow inside these beads without being grazed by zooplankton. For P-limited immobilized cells, however, a lower µm and initial slope of the Monod growth curve µm/Ks were found than for free cells.To study the feasibility of immobilized algae to estimate algal growth potentialin situ in aquatic ecosystems, a series of experiments were conducted in indoor model ecosystems (microcosms) and in a small stream. The use of immobilized algae allowed a continuous registration of algal growth potential integrated over periods with natural fluctuations in the environment. The method of encapsulation of the algae can, however, still be improved. The alginate matrix is exposed to marked degradation by microorganisms when incubated in polluted streams for a period longer than two weeks. The applicability of other types of matrices should be tested.

Predictability and possible mechanisms of plankton response to reduction of planktivorous fish

The predictability of plankton response to reductions of planktivorous fish was investigated by comparing the plankton community in three biomanipulated lakes and ten unmanipulated lakes differing in intensity of fish predation. Data collected on total phosphorus, phytoplankton and zooplankton biomass and share of cyanobacteria and large grazers, as well as specific growth rate of phytoplankton, were further used to test some of the proposed underlying response-mechanisms. In the biomanipulated lakes the algal biomass and share of cyanobacteria decreased, specific growth rate of phytoplankton increased, and zooplankton biomass and share of large grazers increased or remained unchanged. This pattern was largely reflected in the differences in food-chain structure between the unmanipulated lakes with highversus those with low fish predation. The qualitative response to planktivorous fish reduction thus seems largely predictable. The biomanipulated lakes differed, however, in magnitude of response: the smallest hypertrophic, rotenone-treated lake (Helgetjern) showed the most dramatic response, whereas the large, deep mesotrophic lake (Gjersjoen), which was stocked with piscivorous fish, showed more moderate response, probably approaching a new steady state. These differences in response magnitude may be related to different perturbation intensity (rotenone-treatmentversus stocking with piscivores), food-chain complexity and trophic state. Both decreased phosphorus concentration and increased zooplankton grazing are probably important mechanisms underlying plankton response to biomanipulation in many lakes. The results provide tentative support to the hypothesis that under conditions of phosphorus limitation, increased zooplankton grazing can decrease algal biomassvia two separate mechanisms: reduction of the phosphorus pool in the phytoplankton, and reduction of the internal C:P-ratio in the phytoplankton cells.

Retention of nitrogen in small streams artificially polluted with nitrate

A simple method was developed to test hypotheses on nitrogen retention in first-order streams in an agricultural region near Oslo, SE Norway. A gravity-operated system added a nitrate solution to the streams continuously at a constant rate. Water samples were collected at fixed intervals downstream to follow the rate of decline in streamwater nitrate. Repeated sampling allowed calculation of regression lines from experiments with different levels of additions of nitrate.The experiments showed that removal of nitrate generally increased with higher initial nitrate concentration, regardless of temperature (range 8–16 °C). Higher nitrate removal rates were found in a stream polluted by easily degradable organic matter than in a similar stream fed by groundwater.Experiments conducted in indoor channels lined with a layer of stream sediment gave reproducible, exponential rates of nitrate decrease in the recirculated water.The results are discussed in the framework of first-order streams as protective ecotones between agricultural areas and higher-order parts of the watersheds.

Effects of Dursban® 4E and its carrier on three algal species during exponential and P-limited growth

Abstract To study the direct effects of the insecticide Dursban® 4E (a.i. chlorpyrifos) on freshwater algae, toxicity tests were performed with three different species (diatom: Cyclotella sp., green alga: Selenastrum capricornutum and cyanobacterium: Synechoccocus leopoliensis). The testa were carried out with cells growing under non-limited and phosphorus-limited condilions, using free cells and cells immobilized in alginate beads. The carrier of Dursban 4E was tested separately. Dursban 4E had no appreciable effect on the growth of non-limited algae at concentrations relevant for field situations. For P-limited algae, however, significant and dissimilar effects were found. Striking was the growth stimulation of the P-limited green alga at low concentrations of Dursban 4E (from 0.03 mg 1−1). while the P-limited diatom was inhibited from 0.1 mg 1−1. No marked effect was observed for the P-limited cyanobacterium. The carrier substances in Dursban 4E were responsible for the stimulating effect on the green alga. Shifts in algal species composition and increase in biomass — previously observed in freshwater test ecosystems after treatment with Dursban 4E — may be partly the result of direct species-specific effects of the carrier compounds in Dursban 4E on algal growth. The algal cells immobilized in alginate beads reacted in a rather similar way as the free cells to the application of Dursban 4E and its carrier. This property, combined with other advantages of the beads, such as simple handling and the possibility for exposition of algal cells to toxic substances under field conditions, make the beads suitable for biotest purposes.

Fish size distribution and total fish biomass estimated by hydroacoustical methods: a statistical approach

Abstract Ten data sets of fish catches and echosounding recordings from two small Norwegian lakes were used to define the relationship between target strength (TS) and fish length for roach (Rutilus rutilus) and to estimate statistical errors. A maximum likelihood procedure was used, both separately for each data set, and jointly for all data, each with three alternatives regarding degrees of freedom. Results of the six different approaches have been compared with each other and to earlier estimates. Within estimated statistical errors our data confirmed an earlier estimate in the literature, TS = 20 log10L-68, (where L = length) for a community of whitefish (Coregunus lavaretus) and smelt (Osmerus eperlanus). Two different parameter sets in the TS-L relationship were selected for use in biomass calculations. Results showed that fish biomass of pelagic areas in the two lakes varied considerably, depending on pelagic behaviour of roach. The two different parameter sets gave high estimates from 200 to 400 kg ha−1 in Lake Gjersjoen, with standard deviation factors of 1.2 and 1.6 respectively. For Lake Arungen, high estimates were 300–500 and 900–2300 kg ha−1. The main statistical errors seemed to be errors in the parameters of the TS-L relationship, errors due to the TS scale resolution of 2 dB, and errors in measured fish density in different TS classes.