Ecological implications of artificial mixing and bottom-sediment removal for a shallow urban lake, Lake Sheldon, Colorado

This paper describes the results of the investigation of a shallow urban lake before, and a second year after, restoration to determine the effects of aeration and bottom‐sediment removal on the trophic state and environmental variables controlling the lake's phytoplankton, macrovertebrates and zooplankton compositions. The anthropogenic eutrophic process of the lake is characterized by increased nutrient concentrations from urban run‐off inputs. This addition triggers a chain of events that start with a massive increase in the growth of primary producers, as these are generally growth‐limited by nutrients in freshwater ecosystems. Although the lake was artificially mixed, nuisance cyanobacteria were dominant as primary producers the second year after restoration, during the summers of 2004 and 2005, until this population collapsed and were replaced by diatoms after periods of high winds, rainfall and a decreased temperature. The absence of macrophytes in the main basin because of the removal of bottom sediment had a major effect on macroinvertebrates and zooplankton. Daphnia sp. was the dominant zooplankton species before restoration, being replaced by Bosmina sp., possibly because of the lack of refuge by macrophytes. As an alternative, Bosmina sp. used the surface blooms of cyanobacteria in summer as refuge from grazing by planktivorous fish. The greatest disturbance on the macroinvertebrate community richness and evenness was the decline of scrapers and shredders, because of the lack of macrophytes after removal of the lake bottom sediment.     

Waterbirds and nutrient enrichment in Mar Menor Lagoon, a shallow coastal lake in southeast Spain

The Mar Menor, a 135‐km² saline lake, is the largest water surface on the western Mediterranean coast, and an internationally important bird area. It is surrounded by a large irrigated agricultural plain, with dense tourism developments. Although the impacts of these activities on water quality are locally evident, their effects on waterbird populations are poorly known. In the winter 2004–2005, we studied the distribution of four waterbird species (Podiceps cristatus, Podiceps nigricollis, Phalacrocorax carbo and Fulica atra) around the main drainage channel that discharges into the lake, where it was feasible to infer spatial patterns of eutrophication (alongshore and shore centre) from previous environmental surveys. Waterbirds were counted along a stretch of undeveloped shoreline extending southwards from the channel outlet, in contiguous sections, and in bands parallel to the shoreline. Linear mixed models (LMM) indicated the population density increased only markedly for grebes (Podiceps cristatus, Podiceps nigricollis) and coot (Fulica atra) in littoral bands qualifying as eutrophic, but not an alongshore response, with their finescale alongshore distribution being apparently unrelated to nutrient sources. Considering the whole lake, however, the temporal trends and distribution of the wintering populations indicated that waterbirds respond numerically, in a guild‐specific way, to nutrient inputs. Grebes and coot could be a useful ‘two‐stage’ warning signal for potential problem areas affected by similar influences.     

Long-term effect of tourism on the recent upward trend of phosphorus in a shallow alpine lake, Japan

The trend of total phosphorus (TP) concentrations in May-October of the tourist season was analyzed to examine the effect of more than 30 years of tourism on a shallow oligo-mesotrophic lake, Lake Oze-Numa. Annual mean TP concentrations increased rapidly at the rate of 11.6%/yr, from 7.5mg/m³ to 14.4 mg/m³ in 1982-90, and a mesotrophic condition of 23.1 mg/m³ is predicted for the year 2000. TP concentrations at the end of the tourist season were higher than at the beginning due to autumn circulation of the bottom sediment, and correlated with the current year TP loadings from tourists. TP concentrations in May were the lowest for the season, and showed good correlation with previous year concentrations. The recent upward trend of phosphorus concentration was mainly brought about by long-term tourism because 10% of the TP loading by tourists was estimated to accumulate annually in the water column. Phosphorus reduction measures should be taken urgently in such a shallow lake which now shows an accelerated rate of eutrophication.     

Occurrence patterns of alien freshwater turtles in a large urban pond ‘Archipelago’ (Rome,Italy): Suggesting hypotheses on root causes

The patterns of the occurrence and distribution of alien freshwater turtle species in an urban pond archipelago (Rome, Italy) were analysed, with the aim of exploring the role of a set of factors (type of ponds, landscape context, size area, distance from the nearest road) with a generalized linear model approach. A total of 311 ponds subdivided in three types (fountains, small basins, lakes) embedded in different landscape contexts (public parks, private parks, urban areas) at differing distances from the nearest road were sampled. Six non‐native freshwater turtle species in 31 sites were recorded (9.97%). Lakes exhibited the highest occurrence rates of alien freshwater turtles, compared to small basins and fountains. Freshwater turtle species in urbanized areas were only observed in parks (both public and private). In both the public and private parks, the lakes exhibited the highest percentage of occupied sites, with fountains being the lowest. A direct and significant relationship was observed between pond size and species richness. The distance from the nearest road did not appear to affect species richness. A first interpretation of the data from this study facilitated the postulation of two a posteriori hypotheses that should be tested, as follows: (i) the causal process of turtle release is random, and the rate of extinction (and recapture) is higher in smaller ponds, thus producing the observed pattern; and (ii) the turtle release is not random, and people actively select the ponds they consider more suitable for their pet animals. In this study, it appears the lakes were perceived by those who abandon their pets as the most ecologically suitable habitats among other pond types to accommodate the different species of turtles. Knowledge of people's attitudes in regard to releasing pet animals also might assist managers of public green spaces to develop strategies aimed to preserve local biodiversity, and to educate the public about the conservation issue represented by the alien species.