ALBERT

Notes: SUMMARY. 1. Periphyton. measured as particulate phosphorus (PP) and expressed as periphyton PP, growing on vertically oriented substrata (polyvinyl impregnated nylon) under different nutrient loadings, light intensities (exposures), and grazer communities was examined in eight large enclosures (750 m3) where nutrients (N and P) and planktivorous fish (1+yellow perch) were added in a 2x2 factorial design.2. During the first 3 weeks of the experiment (25 June to 15 July), there was a significantly higher accumulation of phosphorus into periphyton (periphyton PP) with fertilization, but fish addition had no effect. During the fourth to seventh weeks (16 July to 12 August), addition of fish was associated with lower abundance of amphipods and chironomids and higher concentration of periphyton PP. In the enclosures without fish, these invertebrates were over 25 times more abundant, and periphyton PP decreased substantially compared to the June-July period. Fertilization increased periphyton PP only at high exposures in the enclosures with fish.3. Exposure had a significant effect on periphyton PP. In the enclosures with fish, high abundance of nanoplankton reduced water transparency, and periphyton PP was lower in the deeper waters which may have been due to limitation by low light. Lower periphyton PP was also observed at the surface on sunny sides of enclosures without fish, and therefore with high water transparency. This pattern may have been due to inhibitory effects of high light intensity.4. Periphyton communities in the enclosures with fish had higher uptake rates for planktonic phosphorus, and lower rates of phosphorus release, suggesting that periphyton with high phosphorus demand may have high internal cycling of assimilated phosphorus.

Notes: SUMMARY. 1 More than 10 years experience with whole lake pelagic manipulation has suggested some general trends applicable to all freshwater pelagic communities and some specific trends related to lake depth.2 Among the general trends is the observation that the trophic cascade is strongly damped. This means that changes in phytoplankton biomass can be assured only when the fish community is strongly manipulated.3 Among the depth related trends is the observation that in shallow lakes, changes in fish community structure are more likely to have cascading impacts on phytoplankton than are changes in deep lakes.4 In shallow lakes, fish removal frequently results in decreased turbidity which is associated with the development of dense macrophyte populations and significant reductions of algal standing stocks. The mechanisms involve: increased grazing by zooplankton, the removal of fish induced bioturbation and nutrient recycling, and direct and indirect macrophyte effects (shading, zooplankton refuges and competition for nutrients).5 In shallow lakes, where planktivore biomass can be regulated and macrophyte development is acceptable, fish biomanipulalions are likely to result in reduced algal populations and improved water quality.6 In deep lakes, where macrophytes are not as important, long-term effects of fish manipulations are strongly dependent upon the probability of non-grazable algal bloom development. This is determined by many factors (chemical, physical and grazer related) which modify the impact that grazers have on phytoplankton biomass.7 In deep lakes, successful fish biomanipulations may only be effective when chemical and physical factors are altered to produce algal species compositions that permit strong top-down control of prey by predators.

Notes: SUMMARY. 1. The abundance of pianktivorous juvenile yellow perch, Perca flavescens, was manipulated in three 750 m3 enclosures in a eutrophic lake.2. There was a significant negative relationship between fish and zoopiankton biomasses. At high fish densities the zooplankton community was dominated by small filter-feeding cladocera. primarily bosmi- nids. At low fish densities the zooplankton community was dominated by large filter-feeding cladocera, primarily daphnids.3. There was no significant relationship between zooplankton and phytoplankton biomasses when considered over the whole experiment but there was a trend towards lower phytoplankton biomass in the enclosure dominated by daphnids during mid-summer.4. We conclude that although planktivorous fish have a strong negative impact on zooplankton community biomass and size structure, the relationship at the next lower trophic level, zooplankton and phytoplankton, is much weaker. Therefore, the biomanipulation of planktivorous fish populations as a management technique to control phytoplankton abundance is largely ineffective.

Notes: Experiences with lake food web biomanipulation have involved three basic biomanipulation types, repeated in two freshwater environments with distinctly different physio-chemical characteristics. The manipulation types are planktivore additions, complete planktivore removals and planktivore control through piscivore addition. The efficacy of these manipulations is strongly dependent on whether the aquatic environments comprise deep lakes that stratify, or shallow lakes and ponds that are not subject to extended periods of summer stratification. The general results of these six biomanipulation categories are that planktivore additions to stratified lakes almost invariably result in reduced grazer biomasses and increased algal densities. Planktivore additions in shallow lakes or ponds also result in more algae, with the modes of action including: increased bioturbation, decreased grazer biomass and reductions in macrophytes. Planktivore removals in deep stratified lakes frequently result in initial food web-mediated reductions in algal standing stocks, but given sufficient time, ungrazable algae often dominate and total algal biomasses return to premanipulation conditions. Planktivore removals in shallow lakes or ponds tend to be quite stable due to initial decreases in bioturbation and increases in macrophyte cover and zooplankton grazing pressure. However, long-term studies suggest that even these initially very successful manipulations may be subject to deterioration due to macrophyte species substitutions, reinvasion by planktivore-benthivores, and algal species substitutions. The emerging consensus is that in most cases ‘some maintenance is required’. Piscivore additions to deep lakes that stratify, seldom result in more than short periods of algal control. Species substitutions among invertebrate predators, the proliferation of ungrazable algae and changes in nutrient recycling, frequently produce new restructured food webs capable of sustaining algal biomasses that rival those observed during premanipulation periods. Piscivore additions in shallow lakes and ponds have been more successful, but only as long as planktivore-benthivore biomass remains low. Success is further enhanced when macrophyte communities remain intact. However, long-term studies suggest that piscivore densities often decline and that piscivores seem unable to maintain the planktivore-benthivore community at the very low levels that are required. Again the emerging consensus is that in most cases, maintenance is required. None of this invalidates the biomanipulation principle. Many biomanipulations have produced quite spectacular initial results and a few have maintained low algal biomasses for several years. Monitoring and maintenance seems a small price to pay. Given that all of the alternatives to biomanipulation (i.e. sewage treatment in all its forms) are much more management-intensive and expensive, and given that biomanipulations can be applied to non-point source situations that cannot be treated with traditional engineering-based methods, the prospect of having to maintain biomanipulations remains, on balance, quite acceptable. In conjunction with nutrient abatement and with the implementation of well planned maintenance programmes, the cost benefits of biomanipulation seem unassailable.

Notes: Abstract Large lakes enclosures were used to examine the influence of nutrient (P, N) enrichment and planktivorous fish (1 + yellow perch) predation on hypolimnetic oxygen depletion. Results were compared to similar data for lakes with high (Lake St. George) and low (Haynes Lake) abundances of planktivorous fish. In both the unfertilized and fertilized enclosures, fish predation on large cladocerans increased the biomasses of pico- and nanoplankton (0.2–20 µm), phytoplankton (chlorophyll a) and total phosphorus (TP), reduced sedimentation, water clarity, and hypolimnetic oxygen concentrations (AHO). Fertilized enclosures without fish had highest TP and sedimentation rates, but the AHO were low. The high planktivore lake had higher pico- and nanoplankton, higher chlorophyll a, reduced water clarity, and lower AHO than the low planktivore lake. Areal hypolimnetic oxygen depletion (AHOD) rates were strongly related with Secchi depth and plankton size-distribution (r 2 = 0.77, and 0.79, respectively), but not as strongly with TP, chlorophyll a, and sedimentation rates (r 2 = 0.25, 0.53, and 0.02, respectively). Such observations are useful in forming a generalized hypothesis that lakes with low planktivory and high water clarity have lower oxygen depletion because 1) plankton that are settling are larger and spend less time in the hypolimnetic water column before reaching the sediment, and therefore undergo less decomposition, and 2) the euphotic depth extends into the hypolimnion and production of oxygen can take place.

Notes: Abstract Horizontal distributions of zooplankton were investigated in two kettle lakes in southern Ontario. In Tory Lake a set of random samples at 1 m depth showed that Skistodiaptomus oregonensis and copepod nauplii were overdispersed (patchy). In Lake St. George a 20 point grid sampled at each of 0.5, 2, 4, and 6 m showed that Polyarthra spp., Keratella cochlearis, Asplanchna spp., Daphnia galeata mendotae, Bosmina longirostris, Eubosmina coregoni and copepod nauplii were all patchy in terms of both vertical and horizontal distributions. Contour diagrams showed that the patches tended to be comprised of unique groups of species. This was confirmed by principal components analysis which showed that Polyarthra spp. and K. cochlearis occurred together, that D. g. mendotae was found in a unispecies patch and that B. longirostris and E. coregoni were together. None of the zooplankton patches correlated with chlorophyll a measurements. A literature review suggests that there are four basic types of patches occurring in lakes and that there are at least 16 identifiable forces which might cause these distributions. The patch types are: I) large scale (〉 1 km diameter), II) small scale, caused by wind-induced water movement, III) Langmuir circulation aggregations and IV) swarms, potentially caused by biotic factors.

Notes: Abstract Four limnocorrals (8 m dia. by 15 m deep) located at Lake St. George, Ontario, Canada; were used to examine the interactions between planktivorous fish, crustacean zooplankton (notably Daphnia galeata mendotae), and phytoplankton. During the spring, in all of the limnocorrals, high Daphnia biomasses were correlated with increased water transparency and a spring ‘clear-water’ phase. However, as the summer progressed, the relationship between Daphnia biomass and phytoplankton abundance became more complex and less predictable. Investigation of these interactions suggested four conclusions. (1) During late July and throughout August and September, water transparency decreased and algal cell counts increased. In 3 of 4 limnocorrals, deterioration in water quality occurred 3–5 weeks before zooplankton (and Daphnia populations) declined. In all cases decreased transparency was associated with increased concentrations of algal cells (Gloeococcus) that were poor food sources for Daphnia. These results suggested that decreased water transparency was not ‘caused by’ decreases in Daphnia biomass. (2) Taken together, data from all of the limnocorrals showed no correlation between the magnitude of July Daphnia biomasses and the percentage of ‘poor food source’ algae that were observed in August. This suggested that grazer effects were not necessary for the onset of summer ‘poor food source’ algal blooms. (3) In two limnocorrals, there was a positive correlation between increased Daphnia mortality and the onset of ‘poor food source’ blooms. In the other two limnocorrals there was no correlation. In all limnocorrals there was no correlation between decreased Daphnia reproductive capacity and ‘poor food source’ blooms. These data suggested that blooms of ‘poor food source’ algae were not necessary for the collapse of Daphnia populations. (4) In all 4 limnocorrals there was a strong correlation between the time that 0+ yellow perch planktivores reached biomasses of 30–50 kg ha−1 and the collapse of Daphnia populations Species and size selection was also observed. These results suggested that for this set of limnocorral experiments, fish biomasses in the 30–50 kg ha−1 were responsible for the collapse of Daphnia populations in the summer.

Notes: Abstract The uptake of monomethylmercury (mmHg) from water by larvalChaoborus americanus was studied. When exposed to aqueous mmHg for 6 days, the pattern of uptake was sigmoidal. The concentration of mmHg in the exposure water, however, was subject to demethylation and volatilization and therefore decreased over time. A model of passive diffusive uptake underestimated the uptake constant while a bioenergetics-based uptake model provided a reasonable estimate. The data suggest that uptake is not merely a passive diffusive process but may be coupled with oxygen uptake.