Contributions of coastal and watershed energy sources to secondary production in a Northeastern Pacific Estuary

Abstract

We examined the relationship between variation in origin of organic matter and benthic secondary production in a shallow, macrotidal estuary on the United States Pacific Northwest coast, Willapa Bay, Washington. Spatial variation in energy sources and benthic productivity were investigated at both local (vertical height and cross-bank components) and regional (sites within the bay) scales. We determined the stable carbon isotope ratios of oysters (Crassostrea gigas) to evaluate marine versus terrestrial energy sources, compared growth rates of oysters, and made time series measurements of physical variablest at estuarine channel and intertidal stations. The stable carbon isotope ratios of oysters ranged from −22‰ in inner portions of the estuary to −18‰ near the mouth and oysters grown on the substrate surface were enriched in δ13C relative to those grown in the water column. These patterns are consistent with terrigenous inputs away from the estuary mouth and benthic microalgae in the diets of on-bottom oysters. The highest oyster growth was found at an inner estuary site where riverine inputs are relatively high and coincided, with high ammonium in the water column. However, for most sites in Willapa Bay, oyster growth actually declined away from the estuary mouth. Reducing the time available for feeding by transplanting oysters higher in the intertidal zone had significant negative effects on growth(e.g., a reduction of 27–35% over 0.5 m). Despite the fact that oysters grown on-bottom had access to different resources than those in the water column, their growth was slower at amy given tidal elevation, which may be due to on-bottom competition with other suspension feeders, boundary layer effects, or interference from turbidity. In a practical sense, oyster growers have been adjusting to allochthonous energetic support of food webs in Willapa Bay for more than a century, because they have traditionally moved oysters from southern parts of the bay where recruitment is relatively high to beds where market-size oysters can be grown closer to the mouth. This study provides mechanistic support for these practices and suggests that climatic events on a variety of temporal scales (Pacific Decadal Oscillation, upwelling events) could have economic consequences for aquaculture.