Tidal exchange, bivalve grazing, and patterns of primary production in Willapa Bay, Washington, USA

Abstract

Willapa Bay, Washington, USA, is a shallow, coastal-plain, upwelling-influenced estuary where Pacific oysters Crassostrea gigas are intensively cultivated. CTD transect data show that in the long-term average over the May to September growing season, Willapa Bay is a sink for oceanic phytoplankton, not a net exporter: as the tidal circulation stirs ocean water into the estuary, chlorophyll concentration declines by 30 to 60% relative to a hypothetical dilution of the ocean end-member. A 3D circulation model (General Estuarine Transport Model, GETM) was modified to include a phytoplankton-like tracer subject to variable intertidal benthic grazing rates. The grazing rate that best reproduces the along-estuary phytoplankton biomass profile agrees, within confidence limits, with an estimate of cultivated-oyster filtration from literature values. Oysters and other intertidal benthic grazers may thus be the primary cause of the net loss of phytoplankton within the estuary in summer. These grazers appear to be within a small factor of their carrying capacity: as bay-total filtration capacity is increased in the model, the chlorophyll intrusion shortens and food intake per individual grazer declines. Nevertheless, only 8 to 15% of the net tidal supply of oceanic phytoplankton is consumed within the estuary. Even in this well-flushed system, the small-scale structure of tidal transport -rather than total oceanic supply -controls overall food availability for the benthos.

Quantifying the historic contribution of Olympia oysters to filtration in Pacific Coast (USA) estuaries and the implications for restoration objectives

“…Previous work has illustrated that full estuary filtration values may be appropriate indicators of large-scale impacts on seston within estuaries with long residence times which are dominated by autochthonous primary productivity, but less appropriate in estuaries with short residence times (Dame and Prins 1998;Dame 2011). While this is the case for many Atlantic coast estuaries, the potential impact of full estuary filtration is less well understood for Pacific coast systems, where autochthonous primary productivity is less important and oceanic imports of phytoplankton may dominate (Banas et al 2007).…”

“…Given this increased abundance, it is likely that the filtration capacity provided by oysters, albeit a different species, may be returning toward historic values in some estuaries. For example, if we consider the estimated population level filtration rate for cultured C. gigas in Willapa Bay from Banas et al (2007), we find that * 15 % of the estuary could be filtered within the estuary residence time, which lies between our (Table 4). While O. lurida restoration is unlikely to lead to large-scale regulation of seston at whole-estuary scales, restoring oyster beds may nevertheless result in significant local impacts on sea grasses.…”

“…Banas and Hickey (2005) show that the residence time in Willapa Bay is highly uneven throughout the estuary, with water in the upper third of the estuary being retained for 3-5 weeks, while there is near full exchange with every tide near the mouth of the estuary. Oysters are also known to recruit best to the regions in the bay with the greatest retention times (Banas et al 2007); therefore, it is possible that filtration by O. lurida could have been a structuring process in the upper portion of the bay historically.…”

The Pacific oyster, Crassostrea gigas, shows negative correlation to naturally elevated carbon dioxide levels: Implications for near‐term ocean acidification effects

“…On the North American Pacific coast, native populations of the oyster Ostrea lurida were unable to sustain extensive harvesting (White et al 2009), and the commercial oyster industry there is supported primarily by cultivation of the nonnative C. gigas, which was introduced near the beginning of the 20th century but has limited naturally sustaining populations (Ruesink et al 2005) because of cold in situ temperatures limiting reproduction of this species and low residence times of water and, consequently, planktonic larvae in many local estuaries (Banas et al 2007). Therefore, the commercial oyster industry is dependent on hatcheries that rear larvae to settlement size before distributing them to the growers.…”

The Impacts of Nonnative Japanese Eelgrass (<i>Zostera japonica</i>) on Commercial Shellfish Production in Willapa Bay, WA

“…Approximately 20% of the intertidal area is utilized for commercial aquaculture of Pacific oysters and Manila clams [10]. In Willapa Bay, these intertidal benthic grazers rely on oceanic phytoplankton as their main food source, with the majority of their growth occurring between May to September [11]. Clam and oyster farmers utilize natural recruitment and hatchery-set seed; it takes three to five years to reach commercial harvest size.…”