MEPS 404:91-108 (2010)  -  DOI: https://doi.org/10.3354/meps08487

ABSTRACT: On the west coast of British Columbia (BC), Canada, the intertidal is under pressure by commercial farming of the non-native Manila clam Venerupis philippinarum. We applied a 3-tiered approach to determine how farming practices were affecting the geochemical attributes of the intertidal. We compared farmed and reference sites with respect to % silt, organic matter, ammonium, phosphorus, iron and manganese in surface (0 to 3 cm) and bulk (0 to 10 cm) sediments. Ammonium porewater profiles and sediment sorption coefficients were determined for 1 of the 3 regions where the greatest intensity of shellfish farming occurred (Tier I). Mesocosm studies were conducted in which intertidal species were manipulated to match the community composition on the farmed as compared to reference sites (Tier II). Microcosm studies were applied to determine how shifts in community structure as observed on farmed versus reference sites and as manipulated in the mesocosm studies, altered the rate of ammonium diffusion to overlying surface waters (Tier III). All geochemical attributes and ammonium sorption coefficients were greater in either surficial and/or bulk sediments of farmed as compared to reference beaches. Tier III studies indicated that the rate of ammonium diffusion from sediments to surface waters was less in microcosms with only bivalves compared to those with mussels, barnacles and bivalves. Tier I and II studies suggest that greater recovery of ammonium and phosphorus is related to increased bivalve density and % plant coverage on farmed versus reference sites. In contrast, greater amounts of organic matter and silt are due to the presence of antipredator nets. Farming practices that ensure well aerated sediments and support a diverse intertidal community would be advised so that important nutrient geochemical cycles are not altered such to be a detriment to the quality of habitat or to interfere with primary productivity.

KEY WORDS: Intertidal · Community composition · Shellfish aquaculture · Nutrients · Organic matter · Nitrogen · Phosphorous · Iron