Decomposition of macrophyte litter in a subtropical constructed wetland in south Florida (USA)

The South Florida Water Management District has constructed large treatment wetlands (stormwater treatment areas (STAs)) to reduce total phosphorus concentrations in agricultural runoff before this water enters the Everglades. An important component of nutrient removal and storage in these systems is incorporation of nutrients into aquatic macrophytes and burial of this biomass in the sediments. However, decomposition of plant biomass before burial returns nutrients to the water column and may reduce STA treatment efficiency. As part of research on biogeochemical control of STA performance, we conducted a summer (July–September) and a long-term (12-month) experiment (February–February) that measured decomposition rates and release of chemical constituents from dominant aquatic macrophytes in a constructed wetland located in south Florida. The rank order of mean decomposition rates was Najas/Ceratophyllum (0.0568 d⁻¹) > Pistia (0.0508 d⁻¹) > Eichhornia (0.0191 d⁻¹) > submerged Typha (0.0059 d⁻¹) > aerial Typha (0.0008 d⁻¹). Summer decomposition rates were generally higher than rates from the long-term experiment, which suggested a temperature effect. Decomposition rates were negatively correlated with litter C:N and C:P molar ratios and cellulose and lignin content and positively correlated with N and P content. There was no significant difference in decomposition rates among sampling stations despite the fact that there was a decreasing gradient in water column inorganic phosphorus and nitrogen concentrations at these sites. Relatively little of the initial P mass remained in the litter of all species, except Typha, by the end of both experiments. First-order decomposition models derived using nonlinear regression generally had explanatory power, i.e. accounted for variance, comparable to more complex decreasing-coefficient models. Decomposition rates for the species examined in this study were within the range of published values when comparisons were made either by species or by plant group.     

Increased growth and recruitment of piscivorous perch, Perca fluviatilis, during a transient phase of expanding submerged vegetation in a shallow lake

1. In this study, we examine how a 7‐year period of expanding submerged stonewort (Chara spp.) vegetation during a shift from turbid to clear water in a shallow lake influenced individual growth and population size structure of perch (Perca fluviatilis). We expected that a shift from phytoplankton to macrophyte dominance and clear water would improve feeding conditions for perch during a critical benthivorous ontogenetic stage, and enhance the recruitment of piscivorous perch. 2. Growth analysis based on opercula showed that growth during the second year of life was significantly higher in years with abundant vegetation than in years with turbid water and sparse vegetation. Growth was not affected during the first, third and fourth year of life. Stable isotope analyses on opercula from 2‐year‐old perch showed that the increase in growth coincided with a change in carbon source in the diet. Stable nitrogen ratio did not change, indicating that the increased growth was not an effect of any change in trophic position. 3. Following the expansion of submerged vegetation, perch size range and abundance of piscivorous perch increased in central, unvegetated areas of the lake. In stands of stoneworts, however, mainly benthivorous perch were caught, and size range did not change with time. 4. Our findings provide empirical support for the notion that establishment of submerged vegetation may lead to increased recruitment of piscivorous perch, because of improved competitive conditions for perch during the benthivorous stage. This is likely to constitute a benthic‐pelagic feedback coupling, in which submerged vegetation and clear water promote the recruitment of piscivorous perch, which, in turn, may increase water clarity through top‐down effects in the pelagic.     

Evaluating effects of weed cutting on water level and ecological status in Danish lowland streams

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Aquatic plants–based risk model for assessment of ecological safety of rivers

In order to ensure the ecological safety in river beds, each project affecting it must be subjected to an evaluation in terms of its implementation results on the river habitats. Technical activities in the river beds can cause irreversible changes in their biocenosis. However, due to the lack of a method that allows us to predict these changes, the decisions made by designers and contractors can be evaluated only after completing the investment. Therefore, it is necessary to elaborate a method to evaluate the environmental safety in regulated and maintenanced rivers. This article presents a model that allows us to predict the changes in vascular aquatic plant communities, which result from technical interference in the river bed. The model was elaborated based on the results of the study performed in 2007–2011 in natural and transformed lowland watercourses from Lower Silesia area (Poland). It takes into account the elements of the watercourse bed ecosystem, which significantly affect the magnitude of the changes in plant communities. Based on this model, it is possible to evaluate the level of ecological risk associated with the execution of maintenance and regulatory works.     

Size spectra and abundance of planktonic ciliates within various habitats in a macrophyte-dominated lake (Eastern Poland)

Body size, community structure, abundance and biomass of ciliates were compared in various stands of macrophytes in a macrophyte-abundant shallow lake in Eastern Poland. Samples were collected in belts of Phragmites, Typha, Ceratophyllum, Elodea, Stratiotes and Chara. Additionally, protozooplankton was collected from the open water zone surrounding the vegetation belts. Differences in numbers of ciliate taxa between micro-sites were statistically significant. The highest numbers were found in Chara and Ceratophyllum stands, lower numbers in Stratiotes and Elodea stands and the lowest in the open water, Phragmites and Typha areas. Ciliate biomass was, like density, significantly higher in submerged macrophytes than in emergent macrophytes and open water zones. Based on differences in macrophyte structure, two groups of habitats with similar patterns of size-related ciliate distribution were distinguished. The first group consisted of two vegetated zones of sparse stem structure (Phragmites and Typha) and the open water zone, the second group comprised submerged macrophyte species, which were more dense and complex. Generally, the abundance of ciliates correlated positively with total suspension solid (TSS) and total organic carbon (TOC) concentrations. In the Chara and Ceratophyllum stands, relations between ciliate numbers, TSS and TOC were stronger.     

Long‐term responses of leaf litter decomposition to temperature,litter quality and litter mixing in plateau wetlands

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Impacts of a submerged plant (Elodea canadensis) on interactions between roach (Rutilus rutilus) and its invertebrate prey communities in a lake littoral zone

1. Using 5‐m² field enclosures, we examined the effects of Elodea canadensis on zooplankton communities and on the trophic cascade caused by 4–5 year old (approximately 16 cm) roach. We also tested the hypothesis that roach in Elodea beds use variable food resources as their diet, mainly benthic and epiphytic macroinvertebrates, and feed less efficiently on zooplankton. Switching of the prey preference stabilises the zooplankton community and, in turn, also the fluctuation of algal biomass. The factorial design of the experiment included three levels of Elodea (no‐, sparse‐ and dense‐Elodea) and two levels of fish (present and absent). 2. During the 4‐week experiment, the total biomass of euplanktonic zooplankton, especially that of the dominant cladoceran Daphnia longispina, decreased with increase in Elodea density. The Daphnia biomass was also reduced by roach in all the Elodea treatments. Thus, Elodea provided neither a favourable habitat nor a good refuge for Daphnia against predation by roach. 3. The electivity of roach for cladocerans was high in all the Elodea treatments. Roach were able to prey on cladocerans in Elodea beds, even when the abundance and size of these prey animals were low. In addition to cladocerans, the diet of roach consisted of macroinvertebrates and detrital/plant material. Although the biomass of macroinvertebrates increased during the experiment in all Elodea treatments, they were relatively unimportant in roach diets regardless of the density of Elodea beds. 4. Euplanktonic zooplankton species other than Daphnia were not affected by Elodea or fish and the treatments had no effects on the total clearance rate of euplanktonic zooplankton. However, the chlorophyll a concentration increased with fish in all the Elodea treatments, suggesting that fish enhanced algal growth through regeneration of nutrients. Thus, our results did not unequivocally show that Elodea hampered the trophic cascade of fish via lowered predation on grazing zooplankton. 5. In treatments with dense Elodea beds (750 g FW m^?2), chlorophyll a concentration was always low suggesting that phytoplankton production was controlled by Elodea. Apparently, the top‐down control of phytoplankton biomass by zooplankton was facilitated by the macrophytes and operated simultaneously with control of phytoplankton production by Elodea.     

Swim or hide: predator cues cause species specific reactions in young fish larvae

The anti‐predator behaviour of first‐feeding (9 mm total length) hatchery‐reared pike Esox lucius larvae and wild‐caught three‐spined stickleback Gasterosteus aculeatus larvae was studied in the presence of chemical and visual signals from a fish predator. The results clearly showed that both fish species detected the predator by chemical signals alone but the combined chemical and visual signals caused stronger and more diverse reactions. Subsequent to predator detection, their swimming activity decreased, they attacked zooplankton less frequently and spent more time in the vegetation. Fishes differed in their anti‐predator responses. Pike reacted more clearly to chemical signals alone while three‐spined stickleback needed both chemical and visual cues to assess the predation risk. The strongest reaction was the reduction in swimming activity in the three‐spined stickleback (38% decrease) and a decrease in attack rate of the pike (39% decrease), but only when a refuge was available. Pike were more dependent on the vegetation cover showing almost no anti‐predator responses in the absence of a refuge. In addition, there was a difference in the refuge use of three‐spined stickleback between different macrophytes, indicating a complex or dense structure, which was difficult to penetrate or chemical excretion in one of them.     

Levels of organization in biology: on the nature and nomenclature of ecology's fourth level

Viewing the universe as being composed of hierarchically arranged systems is widely accepted as a useful model of reality. In ecology, three levels of organization are generally recognized: organisms, populations, and communities (biocoenoses). For half a century increasing numbers of ecologists have concluded that recognition of a fourth level would facilitate increased understanding of ecological phenomena. Sometimes the word "ecosystem" is used for this level, but this is arguably inappropriate. Since 1986, I and others have argued that the term "landscape" would be a suitable term for a level of organization defined as an ecological system containing more than one community type. However, "landscape" and "landscape level" continue to be used extensively by ecologists in the popular sense of a large expanse of space. I therefore now propose that the term "ecoscape" be used instead for this fourth level of organization. A clearly defined fourth level for ecology would focus attention on the emergent properties of this level, and maintain the spatial and temporal scale-free nature inherent in this hierarchy of organizational levels for living entities.