A review of typhloplanid flatworm ecology, with emphasis on pelagic species

Phytoplankton biomass is compared for 16 temperate and 5 tropical lakes that are sufficiently deep to develop seasonal stratification. The ratio of annual mean to maximum biomass averages 0.45 for tropical lakes and is significantly lower for temperate lakes (0.36). Seasonal (3‐month peak) ratios of mean to maximum biomass (~0.7) do not differ for oligotrophic tropical and temperate lakes, but the ratios for temperate lakes fall below those of tropical lakes at higher trophic states. Tropical lakes of given trophic state have a higher minimum biomass than temperate lakes of the same trophic state; both temperate and tropical lakes show a strong increase in minimum biomass with increasing maximum biomass. The relative range of annual variation for temperate lakes is ~40% greater than for tropical lakes. Differences in the stability of layering and in the duration of seasonal mixing probably explain much of the difference in variability of biomass for tropical and temperate lakes that stratify.

The major classes of tropical lakes include shallow, lowland lakes; deep, tertiary lakes; high altitudinal lakes; rainforests lakes; and man-made lakes at all latitudes and altitudes. Basic ecological processes are similar in temperate and tropical lakes, including grazing, competition, predation and abiotic adaptation. Small tropical lakes of intermediate age are probably not biotically more complicated than similar-sized temperate lakes. The structure of the areas of adaptative radiation and the dispersal ability of the species are important for the present distribution of taxa. Fish play a key role in the tropics since many species both consume zooplankton and compete with them for algal and pelagic sestonic food. This important co-evolu-tion between fish and algae, leaving a fraction of the algal community with a predation refuge, may have decreased the ability of zooplankton to exploit algae. In addition, heavy predation from juvenile and adult fish may greatly simplify the zooplankton community, and have resulted in the scarcity of Cladocera, notably the efficient filter-feeder Daphnia. Little is known of possible physiological constraints to cladoceran distribution, however. Thus similar co-evolution as hypothesized between fish and algae seems not to have occurred to such a great extent between fish and zooplankton. Diurnal patterns in habitat selection of fish may also influence nutrient re-distribution in the tropics as in many temperate lakes. Serious environmental problems threaten tropical lakes, including eutrophication, clear-cutting of the rain forest, unwise introduc-tion of new species not adapted to prevailing conditions, overfishing, extensive use of biocids, and probably acidic rain in areas with poorly buffered waters. Important processes in tropical lakes could be elucidated by concentrating research upon the fate of phytoplankton successional production, involving competition, grazing, sinking, fungi and bacterial attack. Co-evolution of fish and algae should be further investigated as it could in part explain the general scarcity and simplicity of the zooplankton community. Limnocorral experiments should also be used for further assessing processes in tropical lakes.

Summary1. 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.

<p>High mountain lakes are extreme freshwater ecosystems and excellent sentinels of current global change. They are likely among the most comparable ecosystems across the world. The largest contrast occurs between lakes in temperate and tropical areas. The main difference arises from the seasonal patterns of heat exchange and the external loadings (carbon, phosphorus, metals). The consequence is a water column structure based on temperature, in temperate lakes, and oxygen, in tropical lakes. This essential difference implies that, in tropical lakes, one can expect a more sustained productivity throughout the year; a higher nutrient internal loading based on the mineralization of external organic matter; higher nitrification-denitrification potential related to the oxyclines; and a higher metal mobilization due to the permanently reduced bottom layer. Quantifying and linking these and other biogeochemical pathways to particular groups of organisms is in the current agenda of high-mountain limnology. The intrinsic difficulties of the taxonomic study of many of the organisms inhabiting these systems can be now overcome with the use of molecular techniques. These techniques will not only provide a much less ambiguous taxonomic knowledge of the microscopic world, but also will unveil new biogeochemical pathways that are difficult to measure chemically and will solve biogeographical puzzles of the distribution of some macroscopic organism, tracing the relationship with other areas. Daily variability and vertical gradients in the tropics are the main factors of phytoplankton species turnover in tropical lakes; whereas seasonality is the main driver in temperate communities. The study of phytoplankton in high-mountain lakes only makes sense in an integrated view of the microscopic ecosystem. A large part of the plankton biomass is in heterotrophic, and mixotrophic organisms and prokaryotes compete for dissolved resources with eukaryotic autotrophs. In fact, high-mountain lake systems are excellent model ecosystems for applying an investigation linking airshed to sediments functional views. Additionally, the study of the mountain lakes districts as functional metacommunity units may reveal key differences in the distribution of organisms of limited (slow) dispersal. We propose that limnological studies at tropical and temperate high mountain lakes should adhere to a common general paradigm. In which biogeochemical processes are framed by the airshed-to-sediment continuum concept and the biogeographical processes in the functional lake district concept. The solid understanding of the fundamental limnological processes will facilitate stronger contributions to the assessment of the impacts of the on-going global change in remote areas.</p>

Nitrogen (N) availability is one of the principal drivers of primary productivity across aquatic ecosystems. However, the microbial communities and emergent metabolisms that govern N cycling in tropical lakes are both distinct from and poorly understood relative to those found in temperate lakes. This latitudinal difference is largely due to the warm (>20°C) temperatures of tropical lake anoxic hypolimnions (deepest portion of a stratified water column), which result in unique anaerobic metabolisms operating without the temperature constraints found in lakes at temperate latitudes. As such, tropical hypolimnions provide a platform for exploring microbial membership and functional diversity. To better understand N metabolism in warm anoxic waters, we combined measurements of geochemistry and water column thermophysical structure with genome-resolved metatranscriptomic analyses of the water column microbiome in Lake Yojoa, Honduras. We sampled above and below the oxycline in June 2021, when the water column was stratified, and again at the same depths and locations in January 2022, when the water column was mixed. We identified 335 different lineages and significantly different microbiome membership between seasons and, when stratified, between depths. Notably, nrfA (indicative of dissimilatory nitrate reduction to ammonium) was upregulated relative to other N metabolism genes in the June hypolimnion. This work highlights the taxonomic and functional diversity of microbial communities in warm and anoxic inland waters, providing insight into the contemporary microbial ecology of tropical ecosystems as well as inland waters at higher latitudes as water columns continue to warm in the face of global change.IMPORTANCEIn aquatic ecosystems where primary productivity is limited by nitrogen (N), whether continuously, seasonally, or in concert with additional nutrient limitations, increased inorganic N availability can reshape ecosystem structure and function, potentially resulting in eutrophication and even harmful algal blooms. Whereas microbial metabolic processes such as mineralization and dissimilatory nitrate reduction to ammonium increase inorganic N availability, denitrification removes bioavailable N from the ecosystem. Therefore, understanding these key microbial mechanisms is critical to the sustainable management and environmental stewardship of inland freshwater resources. This study identifies and characterizes these crucial metabolisms in a warm, seasonally anoxic ecosystem. Results are contextualized by an ecological understanding of the study system derived from a multi-year continuous monitoring effort. This unique data set is the first of its kind in this largely understudied ecosystem (tropical lakes) and also provides insight into microbiome function and associated taxa in warm, anoxic freshwaters.

A biogeochemical comparison of Lakes Superior and Malawi and the limnological consequences of an endless summer

Fundamental differences between tropical and temperate Great Lakes are the continuously high temperature throughout the water column in tropical lakes and high rates of annual photosynthesis possible under continuously high solar irradiance. These aspects not only lead to permanent stratification and hypolimnetic anoxia in the deepest tropical lakes, but also they have consequences for oxygen concentrations throughout the water column and can dramatically affect the biogeochemical cycles of carbon, nitrogen and phosphorus. Denitrification and enhanced regeneration of phosphorus from metal oxides cause low nitrogen:phosphorus ratios in the deep waters of tropical lakes and create a nitrogen deficit when deep waters mix into surface waters which is met through N-fixation. Comparison of the whole lake nutrient budgets of Lake Superior and Lake Malawi demonstrate the effects of the preferential regeneration of phosphorus in Lake Malawi and loss of nitrogen. In the upper 200 m of permanently stratified Lake Malawi, nitrogen has a residence time of 2 years while in dimictic Lake Superior the nitrogen residence time is over 50 years. This disparity in the residence time indicates that nitrogen is poorly recycled to the mixed layer of Malawi. The physical climate of tropical great lakes affects nutrient biogeochemical cycles differently and imposes biogeochemical characteristics different from temperate lakes on the water quality. In particular, tropical lakes have low nitrogen to phosphorus ratios and chronic, anoxia which will lead to rapid proliferation of nitrogen fixing filamentous cyanobacteria when nutrient loading increases. The chronic hypoxia of tropical lakes will also enhance release of phosphorus bound to metal oxides and allow soil erosion to induce eutrophication in tropical lakes.

Studies suggest that, unlike the situation in temperate lakes, high biomasses of omnivorous fish are maintained in subtropical and tropical lakes when they shift from a turbid phytoplankton-dominated state to a clear water macrophyte-dominated state, and the predation pressure on large-bodied zooplankton therefore remains high. Whether this reflects a higher degree of herbivory in warm lakes than in temperate lakes is debatable. We combined food web studies using stable isotopes with gut content analyses of the most dominant fish species to elucidate similarities and differences in food web structure between a clear water macrophyte-dominated basin (MDB) and a turbid phytoplankton-dominated basin (PDB) of Huizhou West Lake, a shallow tropical Chinese lake. The δ13C–δ15N biplot of fish and invertebrates revealed community-wide differences in isotope-based metrics of the food webs between MDB and PDB. The range of consumer δ15N (NR) was lower in MDB than in PDB, indicating shorter food web length in MDB. The mean nearest neighbor distance (MNND) and standard deviation around MNND (SDNND) were higher in MDB than in PDB, showing a markedly low fish trophic overlap and a more uneven packing of species in niches in MDB than in PDB. The range of fish δ13C (CR) of consumers was more extensive in MDB than in PDB, indicating a wider feeding range for fish in MDB. Mixing model results showed that macrophytes and associated periphyton constituted a large fraction of basal production sources for the fish in MDB, while particulate organic matter (POM) contributed a large fraction in PDB. In MDB, the diet of the dominant fish species, crucian carp (Carassius carassius), consisted mainly of vegetal matter (macrophytes and periphyton) and zooplankton, while detritus was the most important food item in PDB. Our results suggest that carbon from macrophytes with associated periphyton may constitute an important food resource for omnivorous fish, and this may strongly affect the feeding niche and the strength of the top-down trophic cascade between fish and zooplankton in the restored, macrophyte-dominated basin of the lake. This dual effect (consumption of macrophytes and zooplankton) may reduce the chances of maintaining the clear water state at the prevailing nutrient levels in the lake, and regular removal of large crucian carp may therefore be needed to maintain a healthy ecosystem state.

Temporal patterns of phytoplankton biomass and community structure are described for two Kenyan lakes and subsequently compared with patterns reported in other tropical and temperate lakes. Lake Naivasha had a lower and more seasonally variable (10x) biomass, with a seasonal shift between diatoms and blue-greens, while the L. Oloidien biomass was less variable (3.7x) and dominated by blue-greens. Biomass and chlorophyll a were strongly correlated and in turn were coupled to the level of total phosphorus. A total of 143 and 94 taxa were described for L. Naivasha and L. Oloidien, respectively.The comparative analysis showed: a) a paucity of exclusively tropical species; b) that more than 30 percent of the species in two highly saline Kenyan lakes were also present in the two freshwater lakes; c) no evidence for a postulated decline of phytoplankton species abundance with latitude from the temperate zone to the tropics; d) that the low fraction of chrysophyte biomass in tropical lakes is a function of trophy rather than of latitude; e) that the fraction of chlorophyte biomass in tropical lakes is generally higher than in temperate lakes; f) that the proportion of nannoplankton in the two Kenyan freshwater lakes is not different from that in temperate lakes of the same trophy; g) that seasonal or annual biomass oscillations in the tropics are not systematically lower than in the temperate zone; h) evidence for large inter-year difference in the max.:min. biomass ratio in the only tropical lake (L. Naivasha) for which such data are available; i) that an average biomass ratio appears predictable for tropical lakes from the proportion of the sediment surface in contact with epilimnetic water. Overall, no evidence was found that the freshwater tropical phytoplankton composition or dynamics differ in any fundamental fashion from that observed in the temperate lakes during the summer.Keywordsphytoplankton speciesbiomassnannoplanktonseasonal cyclesKenyan lakestropical-temperate comparison

Two of three Kenyan lakes studied between November 1979 and October 1980 have very short 33PO4 turnover times, indicating a high phosphorus (P) demand throughout the year. The P turnover time in Lakes Oloidien and Sonachi is as rapid as in the most P deficient temperate zone lakes. The third lake, Lake Naivasha, has a lower overall P demand and a wide seasonal range, with lowest demand between November 1979 and February 1980 when a P deficiency was unlikely. On an annual basis the Lake Naivasha status is, however, not statistically different from that recorded during the summer in Lake Memphremagog, a generally P-limited temperate zone lake. Lake Naivasha and Lake Oloidien fit well to the line of best fit for the Dillon-Rigler relationship relating total phosphorus (TP) and chlorophyll a derived in temperate zone lakes. Thus, temperate zone models predicting aspects of lake behaviour on the basis of TP may also be applicable to these two tropical lakes. Saline lake Sonachi had not only a short P turnover time but also responded dramatically to the fertilization of enclosures with P. However, it does not fit the TP-chla or the total nitrogen-chla plots from the temperate zone. This suggests that, in this saline lake at least, much of the TP is unavailable to the algae, with some of it in a particulate form that is readily extracted with boiling water. The epilimnetic N:P ratios also characterize lakes Oloidien and Sonachi lakes as highly P deficient and lake Naivasha as more moderately P limited. A single set of measurements in Winam Gulf (Lake Victoria) also showed a rapid P turnover time and thus P limitation, but as in lake Sonachi much of the TP was in a non-algal particulate form. Occasional measurements in three other hypertrophic and saline lakes suggest them to be primarily light limited on the basis of their very high photosynthetic cover. These findings support the hypothesis of a primary P limitation for those lakes not light limited, and contradicts literature suggestions that nitrogen is the primary limiting element in tropical lakes.

The development of ideas leading to the concept of ecological stoichiometry is detailed. Initial work on biogeochemistry of African lakes by P. Kilham led to the development of freshwater diatom physiology to test his silica hypothesis. Further resource experiments on P-limited diatoms by Tilman and his tests of competition led to the discovery that species-specific Si:P ratios were important in determining the outcomes of competition. It became clear that stoichiometry was the clue to understanding plankton community structure. P. Kilham’s continued work on biogeochemistry of African inland waters led to his summary paper about geochemical processes controlling water chemistry and to several joint papers exploring the differences between temperate and tropical lakes. We proposed that biological control of elemental cycles dominates in tropical lakes year round, whereas nutrient cycles in temperate lakes are dominated by physical processes for a large part of the year, resulting in fundamental differences in nutrient regeneration. Following P. Kilham’s death in 1989, S. Kilham continued to test resource ratio theory in collaboration with Theriot and Interlandi in phytoplankton communities in the large lakes of the Greater Yellowstone Ecosystem. Resource ratios of Si, P, N, and light were surprisingly variable and complex in time and space over the growing season. Diatom community structure could be largely explained by each common species’ unique 4D resource signature. It was further observed that the phytoplankton species diversity had a direct significant relationship to the number of potentially limiting resources.

Zooplanktonic communities are known to be amongthe most vulnerable ones with respect to the invasionsof alien species [6]. The invasion of new species intoplanktonic communities often causes changes in thestructure and functions of the entire ecosystem of therecipient body of water. Prediction of such changes is,and has always been, one of the main goals of ecology[5, 13, 14].There are grounds to believe that exploitative com-petition is a potent barrier against the invasion of alienspecies into zooplankton. However, competition as afactor preventing the invasion of new species into com-munities may be expressed differently, depending onthe trophic conditions and predator pressure in thegiven body of water. In this study, we attempted to usesimulation modeling to predict the changes in the spe-cies structure of zooplankton after introduction of apredator under different conditions of food supply.EXPERIMENTALCladocera were the prototype of the model objects.To attain the objectives set in this study, we developeda model of the population dynamics of cladoceransunder the conditions of food depletion. The depen-dences of the species population parameters (fecundity,mortality, and the duration of postembryonic develop-ment) on the food concentration were specified on thebasis of our own observations in natural ecosystemsand laboratory experiments [4], as well as data pub-lished by other researchers [1, 2]. The food concentra-tion was determined as a result of its reproduction andconsumption. The numbers of species were calculatedfor each stage, taking into account the numbers of newindividuals entering the given stage and individualsleaving it by transiting to the next stage or dying. Themodel included a delay of the response of populationparameters to changes in food concentration, whichimitated the storage of nutrients in individuals. We tookinto account that the functions of the dependence ofpopulation parameters on food concentration changedwith the body size and age of individual animals. Theequations used in the model are published elsewhere[3, 4]. Preliminarily, we created a database on morethan 2000 species whose population parameters variedwithin their actual ranges.The effect of a predator (plankton-eating fish) wassimulated by varying the mortality function. Thetrophic states of waters (oligotrophic, mesotrophic,eutrophic, or hypertrophic) were determined by the rateof food resource reproduction in the body of water. Weperformed two series of simulation experiments: with-out predator pressure and a series where the minimumdeath rate varied depending on both the body size andthe numbers of cladocerans.RESULTS AND DISCUSSIONWe simulated 2000 combinations of five crustaceanspecies randomly selected from the database. In mostof these random combinations or “invasions,” only onespecies (rarely, two species) were eventually left in thecommunity (Fig. 1). In other words, the principle ofcompetitive exclusion was complied with. Thus, thezooplanktonic community should be expected to havepoor species diversity in the absence of a predator.If we introduced a predator into the system, thenumber of coexisting species competing for the sameresource increased. In eutrophic and hypertrophic bod-ies of water, the number of coexisting competitorsincreased to four. Under oligotrophic conditions, apredator proved to be incapable of affecting the numberof coexisting species. Apparently, the competition lead-ing to the exclusion of weaker species is more impor-tant for the formation of the community species compo-sition if food is deficient. Under mesotrophic condi-tions, biodiversity also increased, but less than at higherrates of food reproduction.The average concentration of food in mesotrophicand, especially, eutrophic waters increased after theinvasion of a predator compared to this value in theabsence of a predator (Fig. 2). Apparently, the foodconcentration increased upon the invasion of a predator

The communities of pelagic and benthic decapod crustaceans off Namibia (Southeast Atlantic) were studied. The samples comprised 97 species differing widely in their geographical and depth distribution. The analyses revealed distinct assemblages, with several well-defined boundaries. However, the barriers were different for pelagic and benthic species. The zonation of the pelagic species presented two clearly differentiated communities, an inshore from the coast to about 70 miles (113 km) offshore, largely coinciding with the shelf, and an offshore more than 70 miles off the coast. Both associations were present in the active (September-October) and quiescent (April) upwelling period, although during the active upwelling period the number of species clearly decreased. These results support the existence of different circulation patterns over the shelf and slope separated by a cross-shelf barrier. A third association seemed to be related to the seasonal intrusion of waters from Angola during the quiescent upwelling period. The communities of benthic species were mainly delimited by the depth, although several latitudinal boundaries exist. The bathymetric boundaries were well defined: the main boundary was located around 400 m, separating the shelf and slope-bathyal species. The latitudinal boundaries seemed to be related to different features of the Benguela upwelling and the circulation pattern in the region.

Summary1. The zooplankton often undergoes diel horizontal migration (DHM) from the open water to the littoral of shallow lakes, thus avoiding predators in the former. This behaviour has functional impacts within the lake, as it enhances zooplankton survival, increases their control of phytoplankton and tends to stabilise the clear water state. However, most of the evidence supporting this migration pattern comes from cold north temperate lakes, and more evidence from tropical and subtropical areas, as well as from southern temperate areas, is needed.2. We conducted a field study of the diel horizontal and vertical migration of zooplankton, and the horizontal distribution of potential predatory macroinvertebrates and fish, over two consecutive days in the summer in a temperate lake in the southern hemisphere. We took zooplankton samples at two depths, at three sampling stations (inside beds of aquatic macrophytes, at their edge and in open water) along three transects running from the centre of a bed of Ceratophyllum demersum to open water. At each sampling station, we also took samples of macroinvertebrates and fish and measured physical and chemical environmental variables.3. Zooplankton (pelagic cladocerans, calanoid copepods and rotifers) avoided the shore, probably because of the greater risk from predators there. Larger and more vulnerable cladocerans, such as Diaphanosoma brachyurum and Moina micrura, were two to four times more abundant in open water than at the edge of or inside beds of macrophytes, respectively, by both day and night. Less vulnerable zooplankton [i.e. of medium body size (Ceriodaphnia dubia) or with the ability to swim fast (calanoid copepods)] were distributed evenly between open water and the edge of the plant beds. Small zooplankton, Bosmina huaronensis and pelagic rotifers, showed an even distribution among the three sampling stations. Accordingly, no DHM of zooplankton occurred, although larger organisms migrated vertically inside C. demersum stands.4. Macrophytes contained high densities of predatory macroinvertebrates and fish. The predator assemblage, composed of large‐bodied macroinvertebrates (including odonates and shrimps) and small littoral fish, was permanently associated with submerged macrophytes. None of these groups moved outside the plant beds or changed their population structure (fish) over the diel cycle.5. Submerged macrophyte beds do not represent a refuge for zooplankton in lakes where predators are numerous among the plants, implying a weaker top‐down control of phytoplankton biomass by zooplankton and, consequently, a more turbid lake. The effectiveness of macrophytes as a refuge for zooplankton depends on the associated assemblage of predatory macroinvertebrates and fish among the plants.

Reduced top–down control of phytoplankton in warmer climates can be explained by continuous fish reproduction