Nutrient enrichment and trophic organisation in an estuarine food web

This paper reviews several long-term (30 years) data sets relevant to eutro phication in the Ythan estuary, Aberdeenshire, Scotland. These data sets ar e land-use in the catchment, nutrients in the river and estuary, macro-alga l cover and biomass, mudflat invertebrate abundance and shorebird counts an d distributions. The links between the observed patterns of change in these parameters are explored and the evidence for causal relationships assessed , especially in relation to experimental tests of potentially competing hyp otheses. A likely scenario is proposed involving shifts in agriculture towa rds more nitrogen-demanding crops and a higher rate of application of nitro gen to the land; a consequent increase in nitrogen levels in the river and the estuary associated with an increase in the biomass and distribution of macro-algal mats; reductions in invertebrate densities (especially Corophiu m volutator) in the worst-affected areas of the estuary and increases in ab undance in the upper reaches; an initial increase in the shorebird populati ons followed by a decline and a shift in shorebird distributions towards ar eas less affected by macro-algal mats. Important ecological processes for w hich data are limited or our understanding is poor are identified and the n eed for rigorous testing of hypotheses is highlighted. (C) 1999 Editions sc ientifiques et medicales Elsevier SAS.