Diversity, biomass, and ecosystem processes in the marine benthos

Recent studies in terrestrial, plant‐dominated systems have shown that reductions in diversity can affect essential ecosystem processes, especially productivity. However, the exact form of the relationship between diversity and ecosystem functions remains unknown, as does the relevance of these studies to other systems. We studied the relationships between macroinvertebrate species richness and ecosystem functions in a soft‐bottom, intertidal system. We also considered, as a separate variable, the effects of macroinvertebrate biomass on ecosystem functions. A field experiment was conducted at Blackness, a mudflat in the Firth of Forth, Scotland, United Kingdom, using cages with different mesh sizes (195, 300, and 3000 μm) to establish low, medium, and high species richness treatments through differential colonization of defaunated sediments. Low, medium, and high biomass treatments were established by enclosing differing amounts of ambient sediment in defaunated plots. Other treatments controlled for the effects of defaunation and caging. The experiment ran for six weeks in the summer of 1999. All treatments contained species within the same five main functional groups of macroinvertebrate, but species' identity varied both within and between treatments (thus species richness was considered a random, rather than fixed, variable). A total of 27 macroinvertebrate species were sampled across all treatments; 37% of these occurred in the low, 52% in the medium, and 74% in the high diversity treatments. At the end of the experiment, the following physical variables were measured as indicators of ecosystem functions such as sediment stabilization and nutrient fluxes: sediment shear strength (a measure of sediment cohesiveness), water content, silt/clay content, organic content, redox potential (a measure of anoxia), nitrate, nitrite, phosphate and ammonium fluxes, and community respiration. Changes in biomass and species richness were found to have significant effects on oxygen consumption; these relationships were driven in particular by the presence of the largest species in our study, Nephtys hombergii. All other variables were not significantly affected by the treatments. These results support the null hypotheses of no relationship between ecosystem functions and diversity and biomass. However, our experiment was necessarily limited in both spatial and temporal scale; the implications of this when scaling up to larger scale generalizations are discussed. Our results suggest that diversity/biomass/ecosystem function relationships in the soft sediment benthos are likely to be very complex and may depend more on functional groups than species richness.