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%).     

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.     

Zooplankton contribution to particulate phosphorus and nitrogen in lakes

Based on the recognition of rather constant species-specificelement to dry weight ratios in freshwater zooplankton, poolsof metazoan zooplankton P and N were calculated for 45 Norwegianlakes of varying trophy. On the average zooplankton constituted20.4±12.3% and 4.6±4.3% of particulate P and Nrespectively. The fraction was considerably higher in many oligotrophiclakes, at most >50 and 20% respectively, but declined to15% (P) and 5% (N) in meso- to eutrophic lakes. In general,phytoplankton contribution to particulate P was <50%, leavinga large share of particulate P to bacteria and microzooplankton.The zooplankton proportion of particulate P was only weaklyinfluenced by the predation pressure in terms of fish communitystructure. Zooplankton P is important in the overall lake metabolism,and knowledge of this pool may be used to estimate zooplankton-mediatedloss and regeneration of P and improve total nutrient elementbudgets in lakes.     

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 (Gjersjøen), 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.

     

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 µ _m and initial slope of the Monod growth curve µ _m /K _s 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.Author for correspondence     

Use of immobilized algae for estimating bioavailable phosphorus released by zooplankton

Cells of the green alga Selenastrum capricomuxum were immobilizedin permeable alginate beads to prevent them from being grazedby zooplankton. Algae were able to grow in these beads and wereused as a new technique to estimate bioavailable phosphorus(P) released by zooplankton. P-limited algal cells were encapsulatedin alginate beads and used to measure P-release by Daphnia pulexfeeding on P-saturated and P-limited free algal cells. Daphnidsgrazing on P-saturated cells released 20 times more P availablefor the immobilized algae than animals grazing on P-limitedcells (0.06 versus 0.003 µg P mg^–1 Daphnia-DW h^–1).     

Nutrient Enrichment and Planktonic Biomass Ratios in Lakes

Phosphorus (P) to chlorophyll ratios and zooplankton–phytoplankton (Z:P) biomass ratios were assessed in 400 temperate lakes over a gradient of phosphorus (P) and with different fish communities. Most of the lakes in this survey were oligotrophic, with a median total P of 7.3 μg P L⁻¹. Thus, the survey provided information on food web effects during the early phase of eutrophication. There was no tendency toward a reduced yield of autotrophs per unit of P over the gradient covered in this survey. The zooplankton yield per unit of P or chlorophyll a decreased slightly with increased nutrient concentrations, and Z:P biomass ratios decreased with fish community classes, reflecting increased fish predation pressure. However, the variability in biomass ratios within a given range of P and fish class was some 100 times higher than the difference over the gradients. This finding suggests that lake-specific properties, community composition, and food quality are by far the most important determinants of biomass ratios and probably also trophic efficiency in lakes; it further suggests that these factors are superimposed on the general effect of eutrophication, at least up to 30 μg P L⁻¹.     

Biomanipulation and food-web dynamics — the importance of seasonal stability

Responses of phytoplankton biomass were monitored in pelagic enclosures subjected to manipulations with nutrients (+N/P), planktivore roach (Rutilus rutilus) and large grazers (Daphnia) in 18 bags during spring, summer and autumn in mesotrophic Lake Gjersjøen. In general, the seasonal effects on phytoplankton biomass were more marked than the effects of biomanipulation. Primary top-down effects of fish on zooplankton were conspicuous in all bags, whereas control of phytoplankton growth by grazing was observed only in the nutrient-limited summer situation. The effect of nutrient additions was pronounced in summer, less in spring and autumn; additions of fish gave the most pronounced effect in spring. The phytoplankton/zooplankton biomass ratio remained high (10–100) in bags with fish, with the highest ratios in combination with fertilization. The ratio decreased in bags without fish to<2 in most bags, but a real grazing control was only observed in bags with addition ofDaphnia. No direct grazing effects could be observed on the absolute or relative biomass of cyanobacteria (mainlyOscillatoria agardhii). The share of cyanobacteria in total phytoplankton biomass was lowest in summer (7–26%), higher in spring (39–63%) and more than 90% in the autumn experiment. The development of the cyanobacterial biomass was rather synchronous in all bags in all the three experiments. A high biomass ofDaphnia gave no increase in the pool of dissolved nutrients in spring, a slight increase in summer and a pronounced increase in autumn. While a strong decrease in the P/C-cell quota of the phytoplankton was observed from spring to autumn, no effect of grazing or nutrient release could be related to this P/C-status. The experiments indicate that such systems, with high and stable densities of inedible cyanobacteria, are rather insensitive to short-term (3–4 weeks) biomanipulation efforts. This is supported by observations on the long-term development of the lake.

     

Autotroph:herbivore biomass ratios; carbon deficits judged from plankton data

A survey on phytoplankton:zooplankton biomass ratios was performed in 342 Norwegian lakes, covering a wide range in lake size and productivity (total phosphorus: 3–246 g l^–1), but with most localities being oligo- to mesotrophic. Mean phytoplankton biomass was 88 g C l^–1, yet with the majority below 50 g C l^–1and a median of 25 g C l^–1. Total zooplankton biomass displayed a mean and median of 37 and 26 g C l^–1, respectively. Cladocerans were by far the dominant group, making up a median of almost 60% of total zooplankton biomass. Total zooplankton biomass as well as that of major aggregated metazoan taxa (cladocerans, calanoid copepods, cyclopoid copepods and rotifers) all showed a positive, but weak correlation with total phytoplankton biomass. These weak correlations suggest that algal biomass per se is a poor predictor of zooplankton biomass. An average phyto-:zooplankton biomass ratio (C:C) of 2.8 (SD±4.7) was found. 30% of the lakes had a phyto-:zooplankton biomass ratio below unity. While there was no correlation between the phyto-:zooplankton biomass ratio with increasing productivity in terms of P concentration, there was a higher biomass ratio in lakes with high fish predation pressure. The low ratio of phyto-:zooplankton biomass suggest major requirements from non-algal sources of C in the zooplankton diet. The need for dietary subsidizing is also supported by the fact that more than 75% of the lakes had algal biomass less than the estimated threshold for net positive growth of zooplankton, although it should be kept in mind that a high share of picoplankton would imply an underestimation of autotroph biomass in these lakes. Since the C-deficiency apparently is most pronounced in oligotrophic systems, it contradicts the view that the detritus pathways plays a predominant role in highly productive systems only, but while the source of detritus probably is mostly of autochthonous origin in eutrophic lakes, allochthonous detritus will be more important in oligotrophic systems.     

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 mg L-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 predator-sensitive 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 Daphnia than shallow (less than 6 m) lakes. The median percentage of Daphnia peaked at 0.15 mg L-1 in shallow lakes and 0.09 mg L-1 in deep lakes. The assumption that fish are responsible for the unimodality was supported by data on the abundance of potential planktivorous fish (catch net-1 night-1 gill nets with the different mesh sizes [CPUE]). To elucidate the potential cascading effect on phytoplankton, we examined the zooplankton phytoplankton biomass ratio. Even though this ratio was inversely related to CPUE at all TP levels, we found an overall higher ratio in oligotrophic lakes that declined toward low values (typically below 0.2) in hypertrophic lakes. These results suggest that planktivorous fish have a more limited effect on the grazing control of phytoplankton in oligotrophic lakes than in eutrophic lakes, despite similar predator control of large-bodied zooplankton. Accordingly, the phytoplankton yield, expressed as the chlorophyll a-TP ratio, did not relate to CPUE at low TP, but it increased significantly with CPUE at high TP. We conclude that the chances of implementing a successful restoration program using biomanipulation as a tool to reduce phytoplankton biomass increase progressively with increasing TP, but that success in the long term is most likely achieved at intermediate TP concentrations.