No abstract

No abstract

Using the same methodology and identical sites, we repeat a study dating from 1973 and quantify cover of hard coral species, soft corals, sponges, hard substratum and soft substratum, and density of a commercially important reef fish species, the graysby Cephalopholis cruentata, along a depth-gradient of 3-36 m on the coral reefs of Curac¸ao. The objective was to determine the multi-decade change in benthic coral reef cover and structural complexity, and their effect on densities of an associated reef fish species. Total hard coral cover decreased on average from 52% in 1973 to 22% in 2003, representing a relative decline of 58%. During this time span, the cover of hard substratum increased considerably (from 11 to 58%), as did that of soft corals (from 0.1 to 2.2%), whereas the cover of sponges showed no significant change. Relative decline of hard coral cover and of reef complexity was greatest in shallow waters (near the coast), which is indicative of a combination of anthropogenic influences from shore and recent storm damage. Cover of main reef builder coral species (Agaricia spp., Siderastrea siderea, Montastrea annularis) decreased more than that of other species, and resulted in a significant decrease in reef complexity. Although density of C. cruentata was highly correlated to cover of Montastrea and Agaricia in 1973, the loss of coral cover did not show any effect on the total density of C. cruentata in 2003. However, C. cruentata showed a clear shift in density distribution from shallow water in 1973 to deep water in 2003. It can be concluded that the reefs of Curac¸ao have degraded considerably in the last three decades, but that this has had no major effect on the population size of one commercially important coral-associated fish species.

Aim  Urbanization usually leads to biotic homogenization with a decrease in native species and increase in exotic species. We investigated whether local environmental factors in urban water bodies, such as water quality, habitat structure and biotic interactions, influenced the invasion of these systems by exotic macroinvertebrate species.Location  Urban surface water systems in lowlands of the Rhine‐Meuse delta.Methods  Presence and abundance of native and exotic macroinvertebrate species were compared between different urban water types and related to environmental variables with multivariate analysis and spearman’s correlations. Moreover, co‐existence of related native and exotic species was studied.Results  In total nine exotic species were found in the following taxa: Tricladida (1), Crustacea (5), Bivalvia (1) and Gastropoda (2). Taxonomically related native and exotic crustacean species did not seem to be influenced by competition in nutrient‐rich urban waters; most species showed high abundances. Nevertheless, two exotic crustacean species were much more abundant in waters where other crustacean species were absent, possibly filling empty niches. Native species richness and abundance was positively related to environmental heterogeneity in the form of submerged vegetation. The occurrence and abundance of most exotic species were positively related to several eutrophication indicators, such as nitrate, sludge layer and lemnid vegetation.Main conclusions  Exotic species in urban waters were mostly detritivorous or omnivorous and therefore dependent on leaf breakdown. In nutrient‐rich water systems, where food availability was high, exotic crustacean species co‐existed with native crustacean species, while in nutrient‐poor, richly vegetated systems, native Asellidae dominated exotic Asellidae. In the turbid water bodies with very little vegetation, native species richness was low and two exotic crustacean species were relatively abundant in these water systems. Invasibility of urban water systems could be reduced by stimulating the development of submerged and nymphaeid vegetation and decreasing nutrient levels.

Abstract:The water quality of urban drainage ditches in lowlands in the Rhine-Meuse delta was analysed with principal component analysis (PCA) during a dry period and a rain storm, and related to the seepage of polluted river water and effective impervious area (EIA). This was done in order to test the hypothesis that seepage of river water and storm water runoff from impervious areas strongly determine the water quality of urban drainage systems along large lowland rivers.Our analysis revealed that upward seepage of groundwater originating from rivers Rhine and Meuse was positively correlated with nitrate, potassium, sodium and chloride and negatively correlated with alkalinity, calcium, magnesium and iron. EIA was correlated with very few environmental variables (i.e. phosphate, pH and iron in the dry period and iron during the rain storm). Nickel and zinc concentrations generally exceeded the maximum allowable concentrations (MAC), while lead and phosphorus concentrations were just above the nutrient standards and MAC in a few locations during the rain storm. To optimize water quality in urban water systems, attention should be paid to all sources of pollution and not only to EIA. The impact of local groundwater seepage originating from large rivers in lowlands on the chemistry of urban water systems is often underestimated and should be taken into account when assessing water quality and improving water quality status.