Nutrient Retention by Benthic Macrofaunal Biomass of Danish Estuaries: Importance of Nutrient Load and Residence Time

The response of macrobenthic faunal abundance and biomass to nutrient load, and factors that may modify this response are examined by means of inter-estuary comparisons of 14 shallow Danish estuaries. Data for this analysis are the physicochemical and biological variables monitored by local authorities mainly during the period 1989-95. A clear positive effect of nutrient load is demonstrated on benthic biomass, over a wide range of total N-load as the model substance from 2-200 g m ^-2year ^-1. The relationship was curvilinear with a levelling off or even depression of biomass at high total N-load (above c. 35 g m ^-2year ^-1). A mixed chemostat model using total load and hydraulic residence time for estuaries was applied to estimate the load that could be realized into primary production, and consequently enter into benthic production. Two measures, the load corrected for winter export (the realized N-load) and the nutrient pool available for the spring bloom (SBNP) were calculated. The benthic metabolic demand inferred from biomass, assuming an annual P:B ratio of 1 (P, secondary production; B, benthic biomass), was approximately of the same magnitude as both total N-load and realized N-load. A positive correlation was still found between benthic biomass/production and the realized N-load, but the linearity of the regression between them was not improved compared to the corresponding relationship with total load. The best linearly proportional relationship was obtained with the spring bloom N-pool (SBNP) calculated from the chemostat model. Stoichiometry suggested, however, that the spring bloom is of little importance for supporting benthic standing stock in these well flushed estuaries. To explain the strong statistical relationship, despite poor causality, with SBNP, it is suggested that the algorithm behind SBNP reflects the ability of the estuary to retain nutrients in the water mass in the productive period, both as free molecules and included into biological particles. These findings strongly indicate that benthic standing stock system-wide is food limited and indicate the importance of interaction between loading and estuary residence time (flushing) for the outcome of eutrophication. The findings are in agreement with reports that high estuary flushing rate may modify effects of eutrophication, and they deviate from previous studies in shallow coastal areas reporting either no effect, or negative effects, of eutrophication on benthic biomass.