Warming,eutrophication, and predator loss amplify subsidies between aquatic and terrestrial ecosystems

The exchange of organisms and energy among ecosystems has major impacts on food web structure and dynamics, yet little is known about how climate warming combines with other pervasive anthropogenic perturbations to affect such exchanges. We used an outdoor freshwater mesocosm experiment to investigate the interactive effects of warming, eutrophication, and changes in top predators on the flux of biomass between aquatic and terrestrial ecosystems. We demonstrated that predatory fish decoupled aquatic and terrestrial ecosystems by reducing the emergence of aquatic organisms and suppressing the decomposition of terrestrial plant detritus. In contrast, warming and nutrients enhanced cross‐ecosystem exchanges by increasing emergence and decomposition, and these effects were strongest in the absence of predators. Furthermore, we found that warming advanced while predators delayed the phenology of insect emergence. Our results demonstrate that anthropogenic perturbations may extend well beyond ecosystem boundaries by influencing cross‐ecosystem subsidies. We find that these changes are sufficient to substantially impact recipient communities and potentially alter the carbon balance between aquatic and terrestrial ecosystems and the atmosphere.     

Omics-based molecular analyses of adhesion by aquatic invertebrates

Many aquatic invertebrates are associated with surfaces, using adhesives to attach to the substratum for locomotion, prey capture, reproduction, building or defence. Their intriguing and sophisticated biological glues have been the focus of study for decades. In all but a couple of specific taxa, however, the precise mechanisms by which the bioadhesives stick to surfaces underwater and (in many cases) harden have proved to be elusive. Since the bulk components are known to be based on proteins in most organisms, the opportunities provided by advancing ‘omics technologies have revolutionised bioadhesion research. Time-consuming isolation and analysis of single molecules has been either replaced or augmented by the generation of massive data sets that describe the organism's translated genes and proteins. While these new approaches have provided resources and opportunities that have enabled physiological insights and taxonomic comparisons that were not previously possible, they do not provide the complete picture and continued multi-disciplinarity is essential. This review covers the various ways in which ‘omics have contributed to our understanding of adhesion by aquatic invertebrates, with new data to illustrate key points. The associated challenges are highlighted and priorities are suggested for future research.     

Positive feedback in aquatic ecosystems

Aquatic ecosystems offer striking examples of how positive feedback can be integral to the dynamics of complex communities. In particular, microorganisms (bacteria and protozoa) introduce a multitude of positive feedback pathways by rapidly recycling nutrients at the very base of many aquatic food webs. The relatively large magnitude of fluxes being shunted through this 'microbial loop' allows an accumulation of nutrients in localized areas, promotes a general build-up of biomass, and acts as a 'life-support system' in harsh environments. In contrast to customary notions which portray positive feedback effects as undesirable, a reassessment indicates that this 'bootstrapping' can often be advantageous for many organisms.     

Heat-flow measurements in aquatic ecosystems

Heat production by plankton and intertidal benthos reveal anddisprove underlying assumptions about energy flow in aquaticecosystems.     

Succession and soil development in man-made coastal ecosystems at the Baltic Sea

By the creation of Køge Bay Seaside Park in 1978 the opportunity was given to study the establishment of vegetation and primary succession in a man-made coastal area. As the soil consists of marine material from the Baltic Sea, no organic matter or seedbank was present. The first steps of primary succession were followed in 1980, when the initial inventories of flora immigration and soil development were carried out in coastal grasslands and plantings. The surveys were repeated in 1992 and 1993. Except for the planted woody species and a few sown grasses, all other species of plants have reached the area through natural dispersal of diaspores. The total number of species in the permanent plots has increased from 26 in 1980 to 91 in 1993. The results indicate that this number will continue to increase in the coming years until a certain level. Then it will probably decrease as a result of competition from woody species, unless the vegetation is kept in a steady state by disturbances or management. Today the area is very far from the initial situation, and the off-shore barrier has changed towards a landscape dominated by small groves and grasslands of an urban common type.     

Aspartic proteinases in fishes and aquatic invertebrates

1. The literature on molecular properties and physiological role of aspartic proteinases in fishes and aquatic invertebrates has been reviewed. 2. Pepsins have not been detected in invertebrates, and apparently cathepsin D, as well as other cathepsins, act both as digestive and lysosomal enzymes in many of these animals. The molecular properties of invertebrate cathepsin D correspond with cathepsin D in fishes and mammalians. 3. Fishes with a true stomach have pepsinogen secretion. Fish pepsins have higher pH optimum and are less stable in strong acid conditions than mammalian pepsins. They are very efficient at low temperatures, but less thermostable than mammalian pepsins. 4. Many fishes have two significantly different pepsins: Pepsin I and Pepsin II, which digest haemoglobin at a maximal rate in the pH ranges 3-4 and 2-3 respectively. Usually the pI of Pepsin I is in the range 6.5-7, whereas pI of Pepsin II is about 4. 5. Fish Pepsin I and cathepsin D have very similar molecular properties, and a hypothesis proposing that cathepsin D is the ancestor enzyme of aspartic proteinases in higher animals is presented.     

Sulfate reduction in coastal ecosystems

The diversity and activity of dissimilatory Fe(III)-reducing bacteria was investigated in acidic, ochre-precipitating springs on Mam Tor, East Midlands, UK. The springs at this acid rock drainage site are located below a 3000 year old landslip, where biooxidation of exposed pyrite-containing minerals has resulted in the production of metal-laden acidic waters. A diverse microbial community was found downstream in the sediments dominated by Fe(III) minerals, and included close relatives to known acidophilic (Acidimicrobium and Acidiphilium) and neutraphilic (Geobacter and Pelobacter) Fe(III)-reducing bacteria. Analysis by XRD and TEM confirmed the presence of both amorphous and well-defined Fe(III) mineral phases in the sediments including lepidocrocite, goethite and schwertmannite. Microcosm-based experiments demonstrated that the bioavailable Fe(III) was reduced under anaerobic conditions, concomitant with sulphate release. XRD analysis suggested that schwertmannite (an iron sulphate hydroxide) was utilized preferentially by the Fe(III)-reducing bacteria, leading to the release of sulphate. Although the microcosms contained sufficient concentrations of naturally occurring electron donor to sustain significant levels of Fe(III) reduction, this process was stimulated by the addition of glycerol and complex electron donors. Thus, the acidic Fe(III)-containing sediments contain a diversity of DIRBs that can be stimulated by the addition of electron donor as a first step in the reversal of acid rock and acid mine drainage contamination.     

Seasonality in coastal benthic ecosystems

For historical reasons, knowledge about seasonality in the dynamics of marine benthic suspension feeders from temperate areas comes mainly from studies of cold temperate seas. Recent surveys of Mediterranean taxa show different patterns from those observed in cold temperate seas, which are characterized by winter dormancy. In the Mediterranean, summer dormancy predominates among taxa and appears to be related to energetic constraints. Temperature and food availability are crucial to the dynamics of benthic suspension feeders. However, because these factors tend to be positively correlated in cold temperate seas, it is difficult to distinguish between their effects. Such correlation does not occur in Mediterranean ecosystems. The contrast between recent studies in the Mediterranean and in other areas can help to disentangle confounded environmental controls.     

The influence of freshwater-lake subsidies on invertebrates occupying terrestrial vegetation

Studies investigating effects of aquatic-derived resource subsidies have often found large effects on terrestrial systems. Those studies have mostly been performed on effects of subsidies derived from oceanic and riverine systems, and very few have considered effects of subsidies from freshwater lakes. However, since lakes can produce large quantities of emergent aquatic insects that end up on nearby land, it is likely that also freshwater-lake subsidies influence terrestrial systems. We performed sweep-net collections of aquatic and terrestrial invertebrates at varying distances from the shore on vegetation of islands of varying size, in two freshwater lakes in northern Sweden, as well as on the surrounding mainland. We found that the amounts of aquatic insects on terrestrial vegetation decreased with distance from the shore, and that they were the most abundant on small islands, presumably because small islands have a higher perimeter-to-area ratio. Web-building spiders responded positively to the aquatic subsidy by being the most abundant on small islands and by showing a positive relationship with aquatic insect biomass. However, distance from the shore showed no effects on the spiders. Our results strongly support the view that terrestrial systems are subsidized by lakes, and indicate that freshwater-lake subsidies are important for terrestrial invertebrate community structure on adjacent land. Further, our study shows that ecosystems should be treated as interdependent, not as self-contained units, and may as such be important for an increased understanding of the nature and importance of resource flows across ecosystem boundaries.     

Pesticide toxicity endpoints in aquatic ecosystems

To adequately protect aquatic ecosystems from impactby anthropogenic perturbations it is necessary todistinguish what is safe from what is not. Thisreview examines approaches to this problem in relationto primary and secondary effects of pesticides.Understanding nutrient – plankton and plankton –plankton interrelationships on both spatial andtemporal scales is important if secondary or indirecteffects are to be assessed. Before defining ormeasuring a toxicity endpoint, consideration must begiven to whether to use single species or multispeciestests. Each has its strengths and weaknesses and isreviewed. In single species testing, toxicityendpoints can be more clearly defined butextrapolation of effects to an ecosystem is moredifficult than with multispecies testing and can oftenlead to incorrect conclusions. Interpretation ofmultispecies testing results are challenging andnumerical analysis techniques including methods whoseobjectives are inference, classification andordination are required. Conceptual and fuzzy logicmodelling techniques promise a solution to theinterpretation of multispecies tests.     

Microbial diversity-productivity relationships in aquatic ecosystems

Thanks to recent advances in molecular biology, one's knowledge of microbial co-occurrence patterns, microbial biogeography and microbial biodiversity is expanding rapidly. This MiniReview explores microbial diversity-productivity relationships in the light of what is known from the general ecology literature. Analyses of microbial diversity-productivity relationships from 70 natural, experimental, and engineered aquatic ecosystems reveal patterns that are strikingly similar to those that have long been documented for communities of macroorganisms. Microbial ecology and the general science of ecology are thus continuing to converge.     

Quantifying habitat complexity in aquatic ecosystems

1. Many aquatic studies have attempted to relate biological features, such as species diversity, abundance, brain size and behaviour, to measures of habitat complexity. Previous measures of habitat complexity have ranged from simple, habitat‐specific variables, such as the number of twigs in a stream, to quantitative parameters of surface topography, such as rugosity. 2. We present a new video‐based technique, called optical intensity, for assaying habitat complexity in aquatic ecosystems. Optical intensity is a visual, quantitative technique modifiable for any scale or for a nested analysis. We field‐tested the technique in Lake Tanganyika, Tanzania, on 38 quadrats (5 × 5 m) to determine if three freshwater habitats (sand, rock and intermediate) were quantitatively different. 3. A comparison of the values obtained from optical intensity with a previous measure of surface topography (rugosity) showed that the two corresponded well and revealed clear differences among habitats. Both the new measure and rugosity were positively correlated with species diversity, species richness and abundance. Finally, whether used alone or in combination, both measures had predictive value for fish community parameters. 4. This new measure should prove useful to researchers exploring habitat complexity in both marine and freshwater systems.     

Differential endozoochory of aquatic invertebrates by two duck species in shallow lakes

Animal vectors are essential for the movement of invertebrate resting eggs between water bodies. However, differences in habitat preferences and feeding behaviour between bird species may result in variations in the dispersal of invertebrates via these birds, even if the different bird species live in the same lake. To test such effects, faecal samples from Anas platyrhynchos (collected in autumn and spring) and Tadorna tadorna (collected in spring) were cultured in water at 20° C and 12 L: 12 D conditions in order to quantify the resting eggs which could be internally transported by these birds. One half of each faecal sample was initially cultured at a conductivity of 0.6 mS cm⁻¹ and the other half at 6 mS cm⁻¹. 1054 invertebrates hatched from a total of 60 faecal samples, including cladocerans, copepods, ostracods, rotifers and ciliates, with a wide variability among faeces. Autumn yielded a low proportion of samples with hatchlings (12.5%) compared to spring (90%). Significant differences were observed between birds, but not between conductivity treatments. Thus, our results imply different hatching dynamics affected by disperser and season, but most species transported as resting eggs by birds seem to have a wide tolerance to hatch under variable salinity conditions. These differences may largery influence the metacommunity dynamics of lake networks, and could be a key factor to consider in wetland conservation planning.     

Progress in management of coastal ecosystems

The process of coastal zone management in the United States has progressed to the point that it is now possible to embrace whole coastal ecosystems in management programs. The structural and dynamical features of the ecosystems have to be known before management can succeed. Furthermore, it is necessary to disaggregate the systems into subsystems. The following six subsystems have been used as the best compromise between scientific and administrative needs: the watershed terrain, the land drainage system, the coastal basin, the basin floor, the coastal waters, and the ocean. The Apalachicola National Estuarine Sanctuary in Florida is used as a case history of a managed ecosystem.