Size‐based interactions across trophic levels in food webs of shallow Mediterranean lakes

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A comparison of shallow Danish and Canadian lakes and implications of climate change

1. Winter temperatures differ markedly on the Canadian prairies compared with Denmark. Between 1 January 1998 and 31 December 2002, average weekly and monthly temperatures did not drop below 0 °C in the vicinity of Silkeborg, Denmark. Over this same time, weekly average temperatures near Calgary, Alberta, Canada, often dropped below −10 °C for 3–5 weeks and the average monthly temperature was below 0 °C for 2–4 months. Accordingly, winter ice conditions in shallow lakes in Canada and Denmark differed considerably. 2. To assess the implications of winter climate for lake biotic structure and function we compared a number of variables that describe the chemistry and biology of shallow Canadian and Danish lakes that had been chosen to have similar morphometries. 3. The Danish lakes had a fourfold higher ratio of chlorophyll‐a: total phosphorus (TP). Zooplankton : phytoplankton carbon was related to TP and fish abundance in Danish lakes but not in Canadian lakes. There was no significant difference in the ratio log total zooplankton biomass : log TP and the Canadian lakes had a significantly higher proportion of cladocerans that were Daphnia. These differences correspond well with the fact that the Danish lakes have more abundant and diverse fish communities than the Canadian lakes. 4. Our results suggest that severe Canadian winters lead to anoxia under ice and more depauperate fish communities, and stronger zooplankton control on phytoplankton in shallow prairie lakes compared with shallow Danish lakes. If climate change leads to warmer winters and a shorter duration of ice cover, we predict that shallow Canadian prairie lakes will experience increased survivorship of planktivores and stronger control of zooplankton. This, in turn, might decrease zooplankton control on phytoplankton, leading to ‘greener’ lakes on the Canadian prairies.     

Benthic carbon is inefficiently transferred in the food webs of two eutrophic shallow lakes

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Combined stable isotope and gut contents analysis of food webs in plant-dominated, shallow lakes

Summary 1. To determine feeding links between primary producers, invertebrates and fish, stable isotope analyses and gut content analyses of fish were conducted on the components of four shallow, eutrophic to hypertrophic, plant-dominated lakes.
2. Although separation of basal resources was possible, the diets of both fish and invertebrates were broad, comprising food from different compartments (planktonic, epiphytic/benthic), as well as from different trophic levels.
3. Mixing models were used to determine the extent to which periphyton production supported higher trophic levels. Only one species of invertebrate relied upon periphyton production exclusively.
4. Fish density affected the diets of invertebrates. The response was different for planktonic and epiphytic/benthic invertebrates. The proportion of periphyton production in the diets of zooplankton appeared to increase with fish density, whilst it decreased for other invertebrates.
5. As all zooplankton samples were collected in the open water at dusk, these results are further evidence for the diurnal horizontal migration of zooplankton. Although not conclusive, they are consistent with a behavioural response by invertebrates and zooplankton in the presence of fish.     

Trait-dependency of trophic interactions in zooplankton food webs

Anthropogenic change in the abundance or identity of dominant top predators may induce reorganizations in whole food webs. Predicting these reorganizations requires identifying the biological rules that govern trophic niches. However, we still lack a detailed understanding of the respective contributions of body size, behaviour (e.g. match between predator hunting mode and prey antipredator strategy), phylogeny and/or ontogeny in determining both the presence and strength of trophic interactions. Here, we address this question by measuring zooplankton numerical response to fish predators in lake enclosures. We compared the fit to zooplankton count data of models grouping zooplankters based either on 1) body sizes, 2) antipredator behaviour, 3) body size combined with antipredator behaviour or on 4) phylogeny combined with ontogeny (i.e. different life stages of copepods). Body size was a better predictor of zooplankton numerical response to fish than antipredator behaviour, but combining body size and behaviour provided even better predictions. Models based on phylogeny combined with ontogeny clearly outperformed those based on other zooplankton grouping rules, except when phylogeny was poorly resolved. Removing ontogenetic information plagued the predictive power of the highly-resolved (genus-level) phylogenetic grouping but not of medium-resolved or poorly-resolved phylogenetic grouping. Our results support the recent use of phylogeny as a superior surrogate for traits controlling trophic niches, and further highlight the added value of combining phylogeny with ontogenetic traits. Further improvements in our mechanistic understanding of how trophic networks are shaped are bound to uncovering the trophic traits captured by phylogeny and ontogeny, but that currently remain hidden to us.     

Differences in the substrate spectrum of extracellular enzymes in shallow lakes of differing trophic status

Organic matter fluxes and food web interactions in lakes depend on the abilities of heterotrophic microbial communities to access and degrade organic matter, a process that begins with extracellular hydrolysis of high molecular weight substrates. In order to determine whether patterns of enzymatic hydrolysis vary among shallow lakes of different trophic status, we investigated the hydrolysis of six specific organic macromolecules (polysaccharides) in the spring and late summer in four adjacent shallow lakes of eutrophic, oligotrophic, and dystrophic status in coastal North Carolina, USA. The spectrum of enzyme activities detected differed strongly between lakes, with all six polysaccharides hydrolyzed in West Mattamuskeet in May, while only two substrates were hydrolyzed in Lake Phelps in August/September. Differences in the spectrum of enzyme activities, and therefore the capabilities of heterotrophic microbial communities, were likely driven by variations among lakes in primary productivity patterns, sediment–water interactions, and/or water chemistry. Our data represent a first step towards a better understanding of carbon substrate availability and differences in carbon cycling pathways in shallow lakes of different trophic status.     

Patterns of microbial food webs in Mediterranean shallow lakes with contrasting nutrient levels and predation pressures

Hygraula nitens is a New Zealand native moth with aquatic larvae that feed on submerged aquatic plants. The larvae have been mainly observed using native Potamogeton and Myriophyllum species as a food source, although some studies reported larvae feeding on the alien macrophytes Hydrilla verticillata, Lagarosiphon major and Ceratophyllum demersum. Experimental mesocosm studies showed larvae had a major effect on H. verticillata, C. demersum, L. major, Elodea canadensis and Egeria densa. In both no choice and choice experiments H. nitens larvae showed a clear preference for and the highest consumption of C. demersum, while the native macrophyte Myriophyllum triphyllum ranked fourth out of five alien and two native plant species, indicating a preference of the larvae for alien macrophytes. Additional choice experiments using C. demersum, sampled from different waters in NZ, illustrated that there was a clear difference in H. nitens preference for plants based on their source. However although C. demersum had the lowest leaf dry matter content (LDMC) compared with the other macrophytes, neither the LDMC nor leaf carbon, nitrogen, phosphorus or total phenolic contents alone could explain the preferences of H. nitens, and we conclude that food choice is based on a combination of these and/or additional factors.     

Trophic cascades and trophic trickles in pelagic food webs

Prey-dependent models, with the predation rate (per predator) a function of prey numbers alone, predict the existence of a trophic cascade. In a trophic cascade, the addition of a top predator to a two-level food chain to make a three-level food chain will lead to increases in the population size of the primary producers, and the addition of nutrients to three-level chains will lead to increases in the population numbers at only the first and third trophic levels. In contrast, ratio-dependent models, with the predation rate (per predator) dependent on the ratio of predator numbers to prey, predict that additions of top predators will not increase the population sizes of the primary producers, and that the addition of nutrients to a three-level food chain will lead to increases in population numbers at all trophic levels. Surprisingly, recent meta-analyses show that freshwater pelagic food web patterns match neither prey-dependent models (in pelagic webs, ''prey'' are phytoplankton, and ''predators'' are zooplankton), nor ratio-dependent models. In this paper we use a modification of the prey-dependent model, incorporating strong interference within the zooplankton trophic level, that does yield patterns matching those found in nature. This zooplankton interference model corresponds to a more reticulate food web than in the linear, prey-dependent model, which lacks zooplankton interference. We thus reconcile data with a new model, and make the testable prediction that the strength of trophic cascades will depend on the degree of heterogeneity in the zooplankton level of the food chain.     

Effects of temperature, salinity and fish in structuring the macroinvertebrate community in shallow lakes: implications for effects of climate change

Climate warming may lead to changes in the trophic structure and diversity of shallow lakes as a combined effect of increased temperature and salinity and likely increased strength of trophic interactions. We investigated the potential effects of temperature, salinity and fish on the plant-associated macroinvertebrate community by introducing artificial plants in eight comparable shallow brackish lakes located in two climatic regions of contrasting temperature: cold-temperate and Mediterranean. In both regions, lakes covered a salinity gradient from freshwater to oligohaline waters. We undertook day and night-time sampling of macroinvertebrates associated with the artificial plants and fish and free-swimming macroinvertebrate predators within artificial plants and in pelagic areas. Our results showed marked differences in the trophic structure between cold and warm shallow lakes. Plant-associated macroinvertebrates and free-swimming macroinvertebrate predators were more abundant and the communities richer in species in the cold compared to the warm climate, most probably as a result of differences in fish predation pressure. Submerged plants in warm brackish lakes did not seem to counteract the effect of fish predation on macroinvertebrates to the same extent as in temperate freshwater lakes, since small fish were abundant and tended to aggregate within the macrophytes. The richness and abundance of most plant-associated macroinvertebrate taxa decreased with salinity. Despite the lower densities of plant-associated macroinvertebrates in the Mediterranean lakes, periphyton biomass was lower than in cold temperate systems, a fact that was mainly attributed to grazing and disturbance by fish. Our results suggest that, if the current process of warming entails higher chances of shallow lakes becoming warmer and more saline, climatic change may result in a decrease in macroinvertebrate species richness and abundance in shallow lakes.     

Salinity effects on macroinvertebrate assemblages and waterbird food webs in shallow lakes of the Wyoming High Plains

We determined the biomass and community structure of macroinvertebrates (>500 µm) associated with macrophytes, sediments, and unvegetated open water in three oligosaline (0.8 to 8.0 mS cm^–1) and three mesosaline (8.0 to 30.0 mS cm^–1) lakes in the Wyoming High Plains, USA. Total biomass of epiphytic and benthic invertebrates did not change with salinity, but biomass of macroinvertebrate zooplankton in open water was significantly higher in mesosaline lakes. Community composition of invertebrates differed between the two salinity categories: large grazer/detritivores (gastropods and amphipods) were dominant in oligosaline lakes, whereas small planktivores and their insect predators were more prevalent in mesosaline lakes. Both direct physiological effects of salinity, as well as a shift in the form of primary production from macrophytes to phytoplankton, probably explain these changes in community composition. Salinity effects on invertebrate communities appear to be less important to top avian consumers than are costs of osmoregulation.     

Comparison of pelagic food webs in lakes along a trophic gradient and with seasonal aspects: influence of resource and predation

Composition and seasonal dynamics of phytoplankton, bacteria,and zooplankton (including heterotrophic flagellates, ciliates,rotifers and crustaceans) were studied in 55 lakes in NorthernGermany with different trophic status, ranging from mesotrophicto hypertrophic. Mean abundance and biomass of all groups increasedsignificantly with trophic level of the lake, but bacteria andmetazooplankton showed only a weak correlation and a slightincrease with chlorophyll concentration. Composition of phytoplanktonshowed a dominance of cyanobacteria in hypertrophic lakes, whereasthe importance of chrysophytes and dinophytes decreased withan increase in trophic status. Protozoans (heterotrophic flagellatesand ciliates) made up 24% (mesotrophic lakes) to 42% (hypertrophiclakes) of total zooplankton biomass on average, and were dominatedby ciliates (62–80% of protozoan biomass). Seasonally,protozoans can build up to 60% of zooplankton biomass in spring,when heterotrophic flagellates can contribute     

Sensitivity of tropical forests to climate change in the humid tropics of north Queensland

An analysis using an artificial neural network model suggests that the tropical forests of north Queensland are highly sensitive to climate change within the range that is likely to occur in the next 50–100 years. The distribution and extent of environments suitable for 15 structural forest types were estimated, using the model, in 10 climate scenarios that include warming up to 1°C and altered precipitation from –10% to +20%. Large changes in the distribution of forest environments are predicted with even minor climate change. Increased precipitation favours some rainforest types, whereas decreased rainfall increases the area suitable for forests dominated by sclerophyllous genera such as Eucalyptus and Allocasuarina. Rainforest environments respond differentially to increased temperature. The area of lowland mesophyll vine forest environments increases with warming, whereas upland complex notophyll vine forest environments respond either positively or negatively to temperature, depending on precipitation. Highland rainforest environments (simple notophyll and simple microphyll vine fern forests and thickets), the habitat for many of the region’s endemic vertebrates, decrease by 50% with only a 1°C warming. Estimates of the stress to present forests resulting from spatial shifts of forest environments (assuming no change in the present forest distributions) indicate that several forest types would be highly stressed by a 1°C warming and most are sensitive to any change in rainfall. Most forests will experience climates in the near future that are more appropriate to some other structural forest type. Thus, the propensity for ecological change in the region is high and, in the long term, significant shifts in the extent and spatial distribution of forests are likely. A detailed spatial analysis of the sensitivity to climate change indicates that the strongest effects of climate change will be experienced at boundaries between forest classes and in ecotonal communities between rainforest and open woodland.     

Biogeochemical implications of climate change for tropical rivers and floodplains

Large rivers of the tropics, many of which have extensive floodplains and deltas, are important in the delivery of nutrients and sediments to marine environments, in methane emission to the atmosphere and in providing ecosystem services associated with their high biological productivity. These ecosystem functions entail biogeochemical processes that will be influenced by climate change. Evidence for recent climate-driven changes in tropical rivers exists, but remains equivocal. Model projections suggest substantial future climate-driven changes, but they also underscore the complex interactions that control landscape water balances, river discharges and biogeochemical processes. The most important changes are likely to involve: (1) aquatic thermal regimes, with implications for thermal optima of plants and animals, rates of microbially mediated biogeochemical transformations, density stratification of water bodies and dissolved oxygen depletion; (2) hydrological regimes of discharge and floodplain inundation, which determine the ecological structure and function of rivers and floodplains and the extent and seasonality of aquatic environments; and (3) freshwater–seawater gradients where rivers meet oceans, affecting the distribution of marine, brackish and freshwater environments and the biogeochemical processing as river water approaches the coastal zone. In all cases, climate change affects biogeochemical processes in concert with other drivers such as deforestation and other land use changes, dams and other hydrological alterations and water withdrawals. Furthermore, changes in riverine hydrology and biogeochemistry produce potential feedbacks to climate involving biogeochemical processes such as decomposition and methane emission. Future research should seek improved understanding of these changes, and long-term monitoring should be extended to shallow waters of wetlands and floodplains in addition to the larger lakes and rivers that are most studied.     

The trophic state of shallow lakes in The Netherlands

A study has been carried out on the development of an ecological assessment method for shallow lakes in The Netherlands. Analyses of eutrophication characteristics of 93 lakes with, in total, 127 sampling localities gave insight into some of the steering variables, such as total-P, total-N, chlorophyll-a and transparency. On the basis of phytoplankton species composition, three main groups of lakes could be distinguished. The first group, characterized by persistent filamentous Cyanobacteria, had the highest summer means of chlorophyll-a and total-P. The second group was characterized by non-persistent filamentous Cyanobacteria, occurring in low abundancies, and at lower chlorophyll-a and total-P concentrations. The third group of lakes was characterized by the absence or very low abundance of the filamentous Cyanobacteria. As a result of these analyses, criteria for the abatement of eutrophication in shallow lakes could be refined.     

Bottom-up trophic cascades and material transfer in terrestrial food webs

In contrast to top-down trophic cascades, few reviews have appeared of bottom-up trophic cascades. We review the recent development of research on bottom-up cascades in terrestrial food webs, focusing on tritrophic systems consisting of plants, herbivorous insects, and natural enemies, and attempt to integrate bottom-up cascade and material transfer among trophic levels. Bottom-up cascades are frequently reported in various tritrophic systems, and are important to determine community structure, population dynamics, and individual performance of higher trophic levels. In addition, we highlight several features of bottom-up cascades. Accumulation or dilution of plant nutritional and defensive materials by herbivorous insects provides a mechanistic base for several bottom-up cascades. Such a stoichiometric approach has the potential to improve our understanding of bottom-up cascading effects in terrestrial food webs. We suggest a future direction for research by integration of bottom-up cascades and material transfer among trophic levels.     

Mesocosm experiments on nutrient and fish effects on shallow lake food webs in a Mediterranean climate

1. Nutrient and fish manipulations in mesocosms were carried out on food‐web interactions in a Mediterranean shallow lake in south‐east Spain. Nutrients controlled biomass of phytoplankton and periphyton, while zooplankton, regulated by planktivorous fish, influenced the relative percentages of the dominant phytoplankton species. 2. Phytoplankton species diversity decreased with increasing nutrient concentration and planktivorous fish density. Cyanobacteria grew well in both turbid and clear‐water states. 3. Planktivorous fish increased concentrations of soluble reactive phosphorus (SRP). Larger zooplankters (mostly Ceriodaphnia and copepods) were significantly reduced when fish were present, whereas rotifers increased, after fish removal of cyclopoid predators and other filter feeders (cladocerans, nauplii). The greatest biomass and diversity of zooplankton was found at intermediate nutrient levels, in mesocosms without fish and in the presence of macrophytes. 4. Water level decrease improved underwater light conditions and favoured macrophyte persistence. Submerged macrophytes (Chara spp.) outcompeted algae up to an experimental nutrient loading equivalent to added concentrations of 0.06 mg L^?1 PO₄‐P and 0.6 mg L^?1 NO₃‐N, above which an exponential increase in periphyton biomass and algal turbidity caused characean biomass to decline. 5. Declining water levels during summer favoured plant‐associated rotifer species and chroococcal cyanobacteria. High densities of chroococcal cyanobacteria were related to intermediate nutrient enrichment and the presence of small zooplankton taxa, while filamentous cyanobacteria were relatively more abundant in fishless mesocosms, in which Crustacea were more abundant, and favoured by dim underwater light. 6. Benthic macroinvertebrates increased significantly at intermediate nutrient levels but there was no relationship with planktivorous fish density. 7. The thresholds of nutrient loading and in‐lake P required to avoid a turbid state and maintain submerged macrophytes were lower than those reported from temperate shallow lakes. Mediterranean shallow lakes may remain turbid with little control of zooplankton on algal biomass, as observed in tropical and subtropical lakes. Nutrient loading control and macrophyte conservation appear to be especially important in these systems to maintain high water quality.     

Assessment of climate change effects on alpine summit vegetation in the transition of tropical to subtropical humid climate

Mountain ecosystems are globally among the most vulnerable to the impacts of global warming. This study is carried out on high-mountain summits dominated by dwarf bamboo, Yushania niitakayamensis, in the alpine zone of subtropical Taiwan. A total of six summits were selected using a multi-summit approach, following the common protocol of the Global Observation Research Initiative in Alpine Environments (GLORIA). To assess the climate change effects, we used the soil temperature and statistical downscaling precipitation data to analyze the climatic changing patterns, also applied enhanced vegetation index (EVI) and vegetation diversity index to assess the vegetation responses among summits along the timeline. The results showed that the temperature has a significant increase in all summits, especially in winter. The annual chilling days have reduced around 50% from 2010 to 2019, which would imply the extension of plant growing season. The EVI values showed a tendency to increase over time. Furthermore, the long-term trend of EVI had an abnormal drop in 2018. This coincided with low precipitation and extremely high temperature in the early growing season, which may have caused water shortage and a decline in plant growth. Although the temperature was increasing significantly, the species diversity was slightly increasing in the first survey, but significantly decreased in the third survey. We suggest that the increase in temperature and the extension of the growing season might have led to the expansion of the bamboo species Y. niitakayamensis. Its expansion, combined with an extreme climate event could have caused the decline in species diversity.

     

Near-surface enrichment of zooplankton over a shallow back reef: implications for coral reef food webs

Zooplankton were 3–8 times more abundant during the day near the surface than elsewhere in the water column over a 1–2.4 m deep back reef in Moorea, French Polynesia. Zooplankton were also significantly more abundant near the surface at night although gradients were most pronounced under moonlight. Zooplankton in a unidirectional current became concentrated near the surface within 2 m of departing a well-mixed trough immediately behind the reef crest, indicating that upward swimming behavior, rather than near-bottom depletion by reef planktivores, was the proximal cause of these gradients. Zooplankton were highly enriched near the surface before and after a full lunar eclipse but distributed evenly throughout the water column during the eclipse itself supporting light as a proximal cue for the upward swimming behavior of many taxa. This is the first investigation of the vertical distribution of zooplankton over a shallow back reef typical of island barrier reef systems common around the world. Previous studies on deeper fringing reefs found zooplankton depletion near the bottom but no enrichment aloft. In Moorea, where seawater is continuously recirculated out the lagoon and back across the reef crest onto the back reef, selection for upward swimming behavior may be especially strong, because the surface serves both as a refuge from predation and an optimum location for retention within the reef system. Planktivorous fish and corals that can forage or grow even marginally higher in the water column might have a substantial competitive advantage over those nearer the bottom on shallow reefs. Zooplankton abundance varied more over a few tens of centimeters vertical distance than it did between seasons or even between day and night indicating that great care must be taken to accurately assess the availability of zooplankton as food on shallow reefs.     

The toxic effects of rubber contaminants on microbial food webs and zooplankton of two lakes of different trophic status

Rubber is commonly used in recreational equipment and devicesforsampling in lakes, but there have been few studies of theeffectsof rubber on planktonic organisms. We investigated the toxiceffects of rubber on the microbial food webs of a mesotrophiclakeand a eutrophic lake. Lake water was collected by pumping via,(i) a polyvinylchloride hose and, (ii) a rubber hose. Samplesoflake water collected by each method were incubated insitu in4.25 l enclosures for four days. The lake water was sampledbeforeand after incubation to determine the concentrations ofinorganicnutrients, chlorophyll a, microorganisms (bacteria,picophytoplankton, flagellates, ciliates) and zooplankton.In the mesotrophic lake, momentary exposure (ten seconds) oflakewater to the rubber hose significantly lowered theconcentrationsof chlorophyll a, bacteria, picophytoplankton and somespecies of zooplankton (Boeckella hamata, Bosmina,androtifers), relative to those in water exposed to the plastichose;flagellates, ciliates and Ceriodaphnia dubia were notsignificantly affected. In the eutrophic lake, the effects oftherubber hose on components of the microbial food web were muchlesssevere, and were consistent with the lake's high levels ofdissolved organic carbon (DOC), which is known to chelatetoxicmetals in water.