Secondary production estimates from benthic biomass: assessing coastal eutrophication

Increased benthic biomass has been considered a symptom of eutrophication. While recognizing that cohort analyses provide the most rigorous measures of secondary production, biomass estimates with appropriate conversions and caveats can be used to generate informative first approximations. Regular sampling of biomass from a large scale, long-term, monitoring program of a major ocean outfall on the San Pedro Shelf, California has presented the opportunity to assess eutrophication through estimates of secondary benthic production. This account focused on two stations representing an outfall (Station 0) and a reference site (Station C) 6 km west and upcoast of the outfall. Production estimates were based on 260 bottom samples per station, distributed over four seasons (winter, spring, summer, fall), and thirteen years (1985-1997). Macrobenthic production at Stations 0 and C over the thirteen year period was estimated to be 35.3 g/M²/yr and 37.4 g/M² /yr, respectively. Different taxa dominated production at the two stations. Polychaete and molluscan production at Station 0 was 2 and 2.5 times higher, respectively, than that at Station C. In contrast, echinoderm and crustacean production at Station C was 2 and 2.5 times higher than that at Station 0. When San Pedro production figures were compared to elsewhere, they were well within the range of estimates from other shelf areas and estuaries. Local production figures did not support concerns about increased biomass associated with eutrophication and underlined the value of applying readily obtainable biomass data to compute production estimates in the absence of cohort analysis