Epipelic diatoms dominate the microphytobenthos of estuarine sediments, where they play important roles in ecological processes such as primary production, secondary production and sediment stability. Grazing (top-down control) and nutrients (bottom-up control) regulate the biomass and species composition of intertidal benthic diatom assemblages. However, observations of grazing/predation effects on species richness differ under contrasting nutrient conditions. We investigated the interactive effects of grazing, nutrients and temperature and compared the impacts of Corophium volutator and Hydrobia ulvae -2 species that differ in their feeding strategies and bioturbation effects. Diatom assemblages were collected from 2 estuaries (Biezelingsche Ham, Westerschelde, high nutrient, and Zandkreek, Oosterschelde, low nutrient) in The Netherlands that differ in their dominant macrofaunal grazer species. Assemblages were grown in the laboratory without (control) and with grazing activity under different nutrient and temperature regimes. C. volutator exerted a strong regulatory influence on epipelic diatoms by reducing biomass, and preferentially consuming certain dominant taxa, thereby increasing species richness, evenness and diversity. The percentage of epipsammic species increased in the presence of C. volutator, at the expense of Navicula species. Biezelingsche Ham assemblages grazed by C. volutator were not influenced by nutrient or temperature regime, while control assemblages were influenced by temperature. In contrast, differences in the structure of diatom assemblages between the treatments were far less pronounced for H. ulvaegrazed and control Zandkreek assemblages. H. ulvae appeared to be a general consumer, grazing subdominant species. Species richness was greater at low temperature, regardless of nutrient level. Macrofaunal grazing did not predictably increase or decrease species diversity, but could potentially do both, and it may mask the effects of environmental and bottom-up control.
The factors that influence the sediment stability and the transport of estuarine mudflats are not yet fully understood but knowledge of them is essential in coastal engineering applications and pollution ecology studies. The suggestion that variation in predictive models of sediment stability might be due to site-specific characteristics is investigated using data from four estuarine mudflats (Eden Estuary, Scotland, the Biezelingsche Ham, Zandkreek, and Molenplaat mudflats in The Netherlands). These estuaries differ in their environmental conditions, macrofaunal species composition and local features (e.g. Enteromorpha mats, migratory biofilms). Stable and unstable sediments were compared, and mean chlorophyll-a concentrations and granulometry of the sediments were significantly different between the two groups.Step-wise multiple linear regressions were applied to the sediment stability data of all sites to establish the influences on erosion threshold of microphytobenthic biomass, water content, granulometry, organic carbon content and the abundance of dominant macrofaunal species. The stability of each site was influenced by different factors. Sediment stability of the Eden Estuary was affected by the Enteromorpha bloom; Biezelingsche Ham was influenced by the highly migratory nature of the diatom biofilms and the abundance of Corophium volutator; the polychaete worm Arenicola marina had a net negative effect on sediment stability of the Zandkreek; and the Molenplaat was influenced by microphytobenthic biomass. This research highlights the need for site-specific calibration of models and suggests that a universal proxy parameter for sediment stability is unlikely to be obtained.
Ecosystems are under pressure from multiple human disturbances whose impact may vary depending on environmental context. We experimentally evaluated variation in the separate and combined effects of the loss of a key functional group (canopy algae) and physical disturbance on rocky shore ecosystems at nine locations across Europe. Multivariate community structure was initially affected (during the first three to six months) at six locations but after 18 months, effects were apparent at only three. Loss of canopy caused increases in cover of non-canopy algae in the three locations in southern Europe and decreases in some northern locations. Measures of ecosystem functioning (community respiration, gross primary productivity, net primary productivity) were affected by loss of canopy at five of the six locations for which data were available. Short-term effects on community respiration were widespread, but effects were rare after 18 months. Functional changes corresponded with changes in community structure and/or species richness at most locations and times sampled, but no single aspect of biodiversity was an effective predictor of longer-term functional changes. Most ecosystems studied were able to compensate in functional terms for impacts caused by indiscriminate physical disturbance. The only consistent effect of disturbance was to increase cover of non-canopy species. Loss of canopy algae temporarily reduced community resistance to disturbance at only two locations and at two locations actually increased resistance. Resistance to disturbance-induced changes in gross primary productivity was reduced by loss of canopy algae at four locations. Location-specific variation in the effects of the same stressors argues for flexible frameworks for the management of marine environments. These results also highlight the need to analyse how species loss and other stressors combine and interact in different environmental contexts.
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