Algal species composition and phosphorus recycling at contrasting grazing pressure: an experimental study in sub-Antarctic lakes with two trophic levels

1. To test the response of algal communities to altered grazer abundance in lakes lacking efficient predators on herbivores, we performed field and experimental studies in two sub-Antarctic lakes (South Georgia).
2. The number of algal species in these high latitude lakes is low, and all dominant species have grazer-resistant adaptations, including spines in three dimensions ( Staurastrum sp.), large size ( Tribonema sp.), a mucus sheet allowing viable passage through the gut ( Chlamydocapsa sp., Elakatothrix sp.) or ability to recruit individuals from the sediment surface ( Mallomonas sp.).
3. Algal community composition was only slightly changed by experimentally altered grazer abundance, indicating that it was already adapted for a high grazing pressure. Hence, the diets of herbivores were restricted to vulnerable food organisms such as Mallomonas sp. and heterotrophic flagellates in the water column, and to benthic food sources.
4. At high grazer abundance, the concentration of available phosphorus (PO₄-P) in the water was lower than at low grazer abundances, due to inefficient nutrient regeneration by the copepod herbivores. Hence, in lakes where copepods are dominant grazers, algae suffer both directly from grazing and indirectly from reduced nutrient availability.     

Algal species composition and phosphorus recycling at contrasting grazing pressure: an experimental study in sub-Antarctic lakes with two trophic levels

1. To test the response of algal communities to altered grazer abundance in lakes lacking efficient predators on herbivores, we performed field and experimental studies in two sub-Antarctic lakes (South Georgia).
2. The number of algal species in these high latitude lakes is low, and all dominant species have grazer-resistant adaptations, including spines in three dimensions ( Staurastrum sp.), large size ( Tribonema sp.), a mucus sheet allowing viable passage through the gut ( Chlamydocapsa sp., Elakatothrix sp.) or ability to recruit individuals from the sediment surface ( Mallomonas sp.).
3. Algal community composition was only slightly changed by experimentally altered grazer abundance, indicating that it was already adapted for a high grazing pressure. Hence, the diets of herbivores were restricted to vulnerable food organisms such as Mallomonas sp. and heterotrophic flagellates in the water column, and to benthic food sources.
4. At high grazer abundance, the concentration of available phosphorus (PO₄-P) in the water was lower than at low grazer abundances, due to inefficient nutrient regeneration by the copepod herbivores. Hence, in lakes where copepods are dominant grazers, algae suffer both directly from grazing and indirectly from reduced nutrient availability.     

The zooplankton-phytoplankton interface in lakes of contrasting trophic status: an experimental comparison

We report here the results of an experimental study designed to compare algal responses to short-term manipulations of zooplankton in three California lakes which encompass a broad range of productivity (ultra-oligotrophic Lake Tahoe, mesotrophic Castle Lake, and strongly eutrophic Clear Lake). To assess the potential strength of grazing in each lake, we evaluated algal responses to a 16-fold range of zooplankton biomass. To better compare algal responses among lakes, we determined algal responses to grazing by a common grazer (Daphnia sp.) over a range ofDaphnia densities from 1 to 16 animals per liter. Effects of both ambient grazers andDaphnia were strong in Castle Lake. However, neither ambient zooplankton norDaphnia had much impact on phytoplankton in Clear Lake. In Lake Tahoe, no grazing impacts could be demonstrated for the ambient zooplankton butDaphnia grazing had dramatic effects. These results indicate weak coupling between phytoplankton and zooplankton in Clear Lake and Lake Tahoe, two lakes which lie near opposite extremes of lake trophic status for most lakes. These observations, along with work reported by other researchers, suggest that linkages between zooplankton and phytoplankton may be weak in lakes with either extremely low or high productivity. Biomanipulation approaches to recover hypereutrophic lakes which aim only to alter zooplankton size structure may be less effective if algal communities are dominated by large, inedible phytoplankton taxa.     

Responses of cyanobacteria to herbivorous zooplankton across predator regimes: who mows the bloom?

相似文献   

Different Predation Impacts of Two Cyclopoid Species on a Small-sized Zooplankton Community: An Experimental Analysis with Mesocosms

We used mesocosms to analyze predation impacts on the prey populations and prey community structures by two cyclopoid copepod species, the larger Mesocyclops pehpeiensis and the smaller Thermocyclops taihokuensis, who coexist with small-sized herbivorous zooplankton species in a fish-abundant lake. The overall predation impact on the prey populations was stronger for Mesocyclops than for Thermocyclops. Mesocyclops had a strong and less selective impact on the rotifer community but a selective impact on the crustaceans. In contrast, Thermocyclops had a selective predation impact on rotifers but a weak and less selective impact on the crustacean community. As a result, the former predator reduced the diversity of the crustacean community but not the rotifer community, while the latter had an opposite impact on the diversities of the two communities. It has been suggested that fish induce development of a zooplankton community dominated by the small-sized zooplankton species in fish-abundant lakes. Our results demonstrated that cyclopoid copepods altered species composition and diversity of the small-sized zooplankton community in such lakes. Thus, the results have given an important suggestion on the role of the invertebrate predator cyclopoid copepods, which often coexist with fish, that they determine population dynamics and community structures of small-sized zooplankton in fish-abundant lakes.     

Clay-Turbid Interactions May Not Cascade—A Reminder for Lake Managers

Food web management is a frequently used lake restoration method, which aims to reduce phytoplankton biomass by strengthening herbivorous zooplankton through reduction of planktivorous fish. However, in clay‐turbid lakes several factors may reduce the effectivity of food web management. Increasing turbidity reduces the effectivity of fish predation and weakens the link between zooplankton and phytoplankton. Therefore, the effects of fish stock manipulations may not cascade to lower trophic levels as expected. Additionally, in clay‐turbid conditions invertebrate predators may coexist in high densities with planktivorous fish and negate the effects of fish reductions. For instance, in the stratifying regions of the clay‐turbid Lake Hiidenvesi, Chaoborus flavicans is the main regulator of cladocerans and occupies the water column throughout the day, although planktivorous Osmerus eperlanus is very abundant. The coexistence of chaoborids and fish is facilitated by a metalimnetic turbidity peak, which prevents efficient predation by fish. In the shallow parts of the lake, chaoborids are absent despite high water turbidity. We suggest that, generally, the importance of invertebrate predators in relation to vertebrate predators may change along turbidity and depth gradients. The importance of fish predation is highest in shallow waters with low turbidity. When water depth increases, the importance of fish in the top‐down regulation of zooplankton declines, whereas that of chaoborids increases, the change along the depth gradient being moderate in clear‐water lakes and steep in highly turbid lakes. Thus, especially deep clay‐turbid lakes may be problematic for implementing food web management as a restoration tool.     

Top-down control in pelagic systems: a role for invertebrate predation

Limnologists have long recognized the importance of predation in freshwater communities. The majority of study of predator effects has involved vertebrate predators, with emphasis on planktivorous fish. Documented effects of planktivorous fish have been so dramatic that manipulations of their populations are seen by many as potential tools in lake management. However, the success of such manipulations is often less than desired due to the ubiquitous complexity of food webs and the pervasiveness of compensatory responses to food web manipulation. Recently, enormous effort has been applied to the Lake Kinneret pelagic food web in effort to reduced the abundance of the planktivorous Kinneret bleak Acanthobrama terraesanctae and thereby increase the biomass of herbivorous zooplankton in the hopes of increasing water clarity. We compared potential predation pressure on Lake Kinneret herbivorous zooplankton by bleak and the other major zooplankton predators in the lake, the cyclopoid copepods Mesocyclops ogunnus and Thermocyclops dybowskii. We found that, despite having much lower biomass, cyclopoid copepods accounted for a greater portion of the predation mortality on herbivorous zooplankton than bleak. Our results suggest that reductions in predation pressure by bleak will not yield subsequent increases in herbivorous zooplankton biomass. Rather, reductions in bleak predation pressure may allow for increases in cyclopoid copepod abundance and thereby a net increase in predation pressure on herbivorous zooplankton.     

Invasive trout increase the climatic sensitivity of zooplankton communities in naturally fishless lakes

相似文献   

Food-web manipulations influence grazer control of phytoplankton growth rates in Lake Michigan

Stocking piscivorous salmonids in Lake Michigan produced dramaticalterations in food-web structure, including higher numbersof large-bodied zooplankton (especially Daphnia pulicaria),lower summer chlorophyll concentrations and increased watertransparency. Experimental determinations of epilimnetic phytoplanktongrowth rates and of zooplankton grazing rates indicate thatherbivorous zooplankton controlled algal dynamics during thesummer of 1983 because grazers occupied the surface waters throughoutthe day. In 1985, however, both large- and small-bodied Daphniamade approximately equal contributions to total grazer biomass,and all grazers displayed pronounced diel vertical migrations,visiting epilimnetic waters only at night. This prohibited zooplanktonfrom controlling algal dynamics because grazing losses did notexceed phytoplankton growth rates. The changes in zooplanktoncommunity composition and behavior observed in summer 1985 probablyresulted from increased predation by visually orienting planktivorousfish, especially bloater chub (Coregonus hoyi). Effects of food-webmanipulations on phytoplankton dynamics were evident only duringJuly and August. During spring and early summer copepods dominateLake Michigan's zooplankton community. Owing to their smallbody size, copepods are less susceptible to fish predation andexhibit much lower filtering rates than Daphnia. Variabilityin zooplanktivorous fish abundance probably has little effecton phytoplankton dynamics during spring and early summer.     

Invasive Predator, <Emphasis Type="Italic">Bythotrephes</Emphasis>, has Varied Effects on Ecosystem Function in Freshwater Lakes

Bythotrephes longimanus is an invertebrate predator that has invaded the North American Great Lakes and a number of inland lakes, where it preys on crustacean zooplankton. We examined the effect of Bythotrephes on two measures of ecosystem function during a four-month observational study of freshwater lakes on the boreal shield. Bythotrephes-invaded lakes had significantly lower epilimnetic zooplankton abundance and production compared to reference lakes. On average, Bythotrephes consumed 34% of zooplankton production when it was present in lakes. There was some evidence of changes in the timing of zooplankton production, as well as shifts to cooler, less productive habitats, which may lessen the overall effect of the invader on the transfer of energy to higher trophic levels. We experimentally demonstrated a weak trophic cascade where invader predation reduced zooplankton biomass, and subsequently increased phytoplankton growth. However, the response was small in magnitude and not biologically relevant at the whole lake-scale. The most conspicuous effect of Bythotrephes that we measured was a diversion of energy away from native predators at higher trophic levels. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Indirect food web effects of <Emphasis Type="Italic">Bythotrephes</Emphasis> invasion: responses by the rotifer <Emphasis Type="Italic">Conochilus</Emphasis> in Harp Lake,Canada

As a recent invader of North American lakes, Bythotrephes longimanus has induced large changes in crustacean zooplankton communities through direct predation effects. Here we demonstrate that Bythotrephes can also have indirect food web effects, specifically on rotifer fauna. In historical time series data, the densities of the colonial rotifer Conochilus unicornis significantly increased after Bythotrephes invasion in Harp Lake, Ontario. No such changes were observed in a non-invaded reference lake, the nearby Red Chalk Lake. Evidence for two mechanisms explaining the Conochilus increase was examined based on changes to the crustacean zooplankton community over time. Rapid and severe declines in several herbivorous species of cladoceran zooplankton after Bythotrephes detection indicated a decrease in exploitative competition pressure on Conochilus. Secondly, a later and significant decline to virtual extinction of native invertebrate predators (Mesocyclops and Leptodora) could account for the observed Conochilus increase which also began 1–2 years after invasion by Bythotrephes. Ultimately, it appears that both reduced competition followed by a loss of native invertebrate predators were necessary to lead to the large Conochilus densities observed following invader establishment. From this analysis of long-term community data, it appears that Bythotrephes has important indirect, as well as direct, food web effects in newly invaded North American lakes with implications for trophic relationships.     

A comparison between predation effects on zooplankton communities by Neomysis and Chaoborus

Community level effects of predation by two invertebrate predators, the opossum shrimp (Neomysis intermedia), and the larva of the phantom midge (Chaoborus flavicans) were studied and compared. N. intermedia appeared abundantly in the shallow eutrophic Lake Kasumigaura and had a significant impact on the zooplankton community. The predation pressure by Neomysis was highest on cladocerans, followed by rotifers, and finally copepods. At high densities (maximum nearly 20 000 individuals m^–2), Neomysis excluded almost all cladocerans, rotifers and copepods from the lake.Zooplankton communities were established in experimental ponds, into which C. flavicans was introduced. The predator's density was around 1 individual l^–1, and was probably controled by cannibalism. Although Chaoborus larvae feed on various zooplankton species, their predation impact on zooplankton populations was markedly selective. They eliminated medium- and small-sized cladocerans and calanoid copepods from the ponds, but rotifers increased.Although the feeding selectivities of Neomysis and Chaoborus individuals were similar, the predation effects on zooplankton communities by the two predators were different.     

The effect of two scales of habitat architecture on benthic grazing in a river

1. This experiment studied the effects of differing levels of the complexity of substratum architecture at two spatial scales on the distribution and abundance of benthic algae and invertebrates, and the strength of the trophic interaction between invertebrate grazers and algae. Some estimates of the effects on invertebrate colonization rates were also made. 2. Four levels of microhabitat architectural complexity were created using artificial substrata (clay tiles) and placed in Mountain River, Tasmania, in two riffle types (bedrock and boulder-cobble) of differing large-scale substratum complexity. After a colonization period, invertebrate grazers were removed from half the tiles to measure the effects of grazing. Invertebrates on the tiles were also counted and identified. At the end of the experiment, algae were removed from the tiles and analysed for chlorophyll a. 3. Invertebrate grazers did not reduce algal biomass during the experiment, and microhabitat-scale architecture influenced algal biomass more strongly than riffle-scale architecture. Highly complex microhabitat architecture increased algal biomass by providing more surface area, but once standardized for surface area, algal biomass decreased as the complexity of microhabitat architecture increased. 4. Microhabitat-scale architecture was also predominant in determining invertebrate density and the identity of the dominant grazer species. In contrast to algal biomass, invertebrate densities and species density increased with the complexity of microhabitat architecture, suggesting that refuges from flow (and possibly predation) were as important to river invertebrates as the distribution of their food source. 5. Riffle-scale architecture had some effect on the colonization of two slow-moving grazer taxa, but, overall, the colonization processes of slow-moving grazers were determined mostly by the complexity of microhabitat-scale architecture.     

Ctenophore-zooplankton-phytoplankton interactions in Narragansett Bay, Rhode Island, USA, during 1972-1977

Plankton dynamics at a station in lower Narragansett Bay, RIare compared for six summer and fall seasons, 1972–1977.In four of these years, initiation of the summer pulse of thectenophore Mnemiopsis leidyi was accompanied by a rapid declinein zooplankton abundance and a summer phytoplankton bloom. Terminationof the phytoplankton bloom coincided with depleted ctenophoreabundance and increased zooplankton biomass in two of the years.Yearly variations in the summer abundance of the diatom Skeletonemacostatum were positively related to the magnitude of the ctenophorepulse. The magnitude of ctenophore population was related tothe zooplankton biomass present at the start of the pulse. Theserelationships, the timing and magnitude of the plankton eventssuggest that M. leidyi regulated summer zooplankton and phytoplanktondynamics. Ctenophores may control phytoplankton blooms indirectlythrough their predation on herbivorous zooplankton and directlyby the nutrient excretion accompanying such grazing. This evidencethat a planktonic carnivore two trophic steps removed from thephytoplankton regulates the latter's dynamics in NarragansettBay is analogous to reported regulation of benthic algal (kelp)dynamics by the sea otter, lobster and various crabs throughtheir predation on herbivorous sea urchins. The factors responsiblefor the seasonal decrease in ctenophores remain unresolved;ctenophore predators on Mnemiopsis are absent in NarragansettBay. Infection by the vermiform larval anemone, Edwardsia lineata,grazing by the butterfish, Peprilus triacanthus, and changesin food availability, temperature and salinity likewise do notexplain this disappearance.     

Phytoplankton food quality control of planktonic food web processes

We developed a mechanistic model of nutrient, phytoplankton, zooplankton and fish interactions to test the effects of phytoplankton food quality for herbivorous zooplankton on planktonic food web processes. When phytoplankton food quality is high strong trophic cascades suppress phytoplankton biomass, the zooplankton can withstand intense zooplanktivory, and energy is efficiently transferred through the food web sustaining higher trophic level production. Low food quality results in trophic decoupling at the plant-animal interface, with phytoplankton biomass determined primarily by nutrient availability, zooplankton easily eliminated by fish predation, and poor energy transfer through the food web. At a given nutrient availability, food quality and zooplanktivory interact to determine zooplankton biomass which in turn determines algal biomass. High food quality resulted in intense zooplankton grazing which favored fast-growing phytoplankton taxa, whereas fish predation favored slow-growing phytoplankton. These results suggest algal food quality for herbivorous zooplankton can strongly influence the nature of aquatic food web dynamics, and can have profound effects on water quality and fisheries production. Handling editor: D. Hamilton     

Ecosystem consequences of diversity depend on food chain length in estuarine vegetation

Biodiversity and food chain length each can strongly influence ecosystem functioning, yet their interactions rarely have been tested. We manipulated grazer diversity in seagrass mesocosms with and without a generalist predator and monitored community development. Changing food chain length altered biodiversity effects: higher grazer diversity enhanced secondary production, epiphyte grazing, and seagrass biomass only with predators present. Conversely, changing diversity altered top‐down control: predator impacts on grazer and seagrass biomass were weaker in mixed‐grazer assemblages. These interactions resulted in part from among‐species trade‐offs between predation resistance and competitive ability. Despite weak impact on grazer abundance at high diversity, predators nevertheless enhanced algal biomass through a behaviourally mediated trophic cascade. Moreover, predators influenced every measured variable except total plant biomass, suggesting that the latter is an insensitive metric of ecosystem functioning. Thus, biodiversity and trophic structure interactively influence ecosystem functioning, and neither factor's impact is predictable in isolation.     

Effects of habitat complexity on community structure and predator avoidance behaviour of littoral zooplankton in temperate versus subtropical shallow lakes

1. Structural complexity may stabilise predator–prey interactions and affect the outcome of trophic cascades by providing prey refuges. In deep lakes, vulnerable zooplankton move vertically to avoid fish predation. In contrast, submerged plants often provide a diel refuge against fish predation for large‐bodied zooplankton in shallow temperate lakes, with consequences for the whole ecosystem. 2. To test the extent to which macrophytes serve as refuges for zooplankton in temperate and subtropical lakes, we introduced artificial plant beds into the littoral area of five pairs of shallow lakes in Uruguay (30°–35°S) and Denmark (55°–57°N). We used plants of different architecture (submerged and free‐floating) along a gradient of turbidity over which the lakes were paired. 3. We found remarkable differences in the structure (taxon‐richness at the genus level, composition and density) of the zooplankton communities in the littoral area between climate zones. Richer communities of larger‐bodied taxa (frequently including Daphnia spp.) occurred in the temperate lakes, whereas small‐bodied taxa characterised the subtropical lakes. More genera and a higher density of benthic/plant‐associated cladocerans also occurred in the temperate lakes. The density of all crustaceans, except calanoid copepods, was significantly higher in the temperate lakes (c. 5.5‐fold higher). 4. Fish and shrimps (genus Palaemonetes) seemed to exert a stronger predation pressure on zooplankton in the plant beds in the subtropical lakes, while the pelagic invertebrate Chaoborus sp. was slightly more abundant than in the temperate lakes. In contrast, plant‐associated predatory macroinvertebrates were eight times more abundant in the temperate than in the subtropical lakes. 5. The artificial submerged plants hosted significantly more cladocerans than the free‐floating plants, which were particularly avoided in the subtropical lakes. Patterns indicating diel horizontal migration were frequently observed for both overall zooplankton density and individual taxa in the temperate, but not the subtropical, lakes. In contrast, patterns of diel vertical migration prevailed for both the overall zooplankton and for most individual taxa in the subtropics, irrespective of water turbidity. 6. Higher fish predation probably shapes the general structure and dynamics of cladoceran communities in the subtropical lakes. Our results support the hypothesis that horizontal migration is less prevalent in the subtropics than in temperate lakes, and that no predator‐avoidance behaviour effectively counteracts predation pressure in the subtropics. Positive effects of aquatic plants on water transparency, via their acting as a refuge for zooplankton, may be generally weak or rare in warm 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.

     

Refuge availability and sequence of predators determine the seasonal succession of crustacean zooplankton in a clay-turbid lake

The contribution of predators and abiotic factors to the regulation of the biomass and seasonal succession of crustacean zooplankton was studied in Lake Rehtijärvi (southern Finland). Field data in combination with bioenergetics modeling indicated that invertebrate planktivory by Chaoborus depressed cladoceran populations during early summer. In particular, bosminids that generally form the spring biomass peak of cladocerans in stratified temperate lakes did not appear in the samples until July. In July, predation pressure by chaoborids was relaxed due to their emergence period and cladoceran population growth appeared to be limited by predation by planktivorous fish. The effect of fish predation was amplified by reduced refuge availability for cladocerans. The concentration of dissolved oxygen below the epilimnion was depleted, forcing cladocerans to move upward to less turbid and thus more dangerous water layers. The effect of size selective predation by fish resulted in reduced mean size of cladocerans during the period when refuge thickness (thickness of the water layer with oxygen concentration <1 mg l^?1 and water turbidity >30 NTU) was lowest. The results confirmed that in clay-turbid lakes, invertebrate predators could be the main regulators of herbivorous zooplankton even when cyprinid fish are abundant.     

Effects of a top invertebrate predator (<Emphasis Type="Italic">Dytiscus alaskanus</Emphasis>; Coleoptera: Dytiscidae) on fishless pond ecosystems

We investigated the predatory effects of Dytiscus alaskanus, a large predaceous diving beetle, on the biomass, species composition and diversity of fishless pond communities. The effects were tested using presence and absence treatments of D. alaskanus in 24 mesocosms distributed among six ponds. We sampled phytoplankton, zooplankton and macroinvertebrates every two weeks for a six week period. Periphyton was sampled from the mesocosm walls on the final day. Total macroinvertebrate biomass decreased in the presence of dytiscids while species richness was not affected. Macroinvertebrate predators, snails and Gammarus lacustris decreased in the dytiscid treatments. Laboratory feeding experiments confirmed feeding preferences consistent with the mesocosm results. Periphyton biomass was six times greater in the dytiscid enclosures, concomitant with the decreased grazing by gastropods and other invertebrate primary consumers indicating a benthic trophic cascade. Top–down effects of dytiscids on other predatory invertebrates led to increased total zooplankton biomass, largely due to increased abundances of large and small cladocerans. Zooplankton species richness increased in the dytiscid enclosures. Inconsistent with trophic cascade theory, phytoplankton did not respond to top–down effects of D. alaskanus within the study period. Overall, the results show D. alaskanus predation caused trophic effects via two distinct food chains, a dytiscid–snail–periphyton trophic cascade, and a dytiscid–predatory macroinvertebrates–zooplankton partial trophic cascade.