Perspectives on Field Studies and Related Biological Models of Bivalve Growth and Carrying Capacity

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.

Section: Food Competition and Carrying Capacitymentioning

confidence: 99%

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

Banas¹,

Hickey

Newton

et al. 2007

“…Hofmann et al 1992, Grant et al 1993, Herman 1993. To go beyond the identification of general trends and reach true predictive capabilities, such models are developed and subsequently modified on the basis of empirical research.…”

Section: Empirical Modelsmentioning

confidence: 99%

Growth and survival of sea scallops Placopecten magellanicus: effects of culture depth

Emerson¹,

Grant²,

Mallet³

et al. 1994

We combined extensive water sampling with monthly growth measurements of juvenile sea scallops held in cages 0 to 200 cm above the bottom to (1) construct predictive empirical models of shell and soft-tissue growth based on oceanographic variables, and (2) determine whether scallops on or near the bottom can derive a food supplement from resuspended sediment when seasonal phytoplankton production is low. Variation in growth was strongly dependent on depth, but this relationship was not consistent over time or tissue type. In late fall, when phytoplankton biomass was generally low (-1 pg chl I-'), the adductor muscle of scallops on the bottom lost mass (-1.5 mg dry wt d.'), but for scallops held only 20 cm higher in the water column, growth was 2.5 mg d.'. During the winter, softtissue growth on the bottom was significantly lower than that of scallops held above the sediment surface. At this time, there was no variation in shell growth with respect to depth. At the end of the study, soft-tissue weight (excluding muscle tissue) of scallops on the bottom was -40% less than that of scallops growing 250 cm above bottom. Rather than providing an energetic benefit, results suggest that high concentrations of seston near the bottom inhibit growth. Empirical regression models of scallop growth using data from water sampling every 2 wk accounted for up to 68% of growth variation, with temperature and seston quality being the most important predictor variables. Marginal improvements to the model uslng data collected hourly with In situ probes suggest that estimates of food supply should be corrected, i.e. reduced, when high flows or high seston concentrations limit filtration rates. In addition, results indicate that attention to the magnitude and variation of predictor variables without consideration of their seasonal coherence may be a primary factor limiting the ability to construct truly predictive models of bivalve growth.

“…Mussels are an important ecological component of rocky shore communities around the world (Dayton 1971, Menge 1976, Frechette & Bourget 1985, Suchanek 1985, Griffiths & Hockey 1987, Grant et al 1993, Dye et al 1994, and often show differences in population structure with different degrees of wave action. This may be a species-or site-specific effect.…”

Section: Introductionmentioning

confidence: 99%

Effect of wave exposure on growth and mortality rates of the mussel Perna perna: bottom-up regulation of intertidal populations

McQuaid¹,

Lindsay²

2000

104

The effect of wave exposure on the growth of low-shore populations of Perna perna (Linnaeus) was determined using 3 independent approaches at 6 sites on the south coast of South Africa: (1) Individual marking; notches were filed on the growing edges of mussels in situ. After 111 d, growth from the mark to the new growing edge was measured. (2) Internal shell-growth bands; mussels from the mark-recapture experiment were used to establish that growth bands are laid down tidally. Unmarked individuals were then used to measure growth over the last 60 tides. (3) Shepherd's length-composition analysis (SLCA); growth rates were determined from length-frequency distributions in 11 samples collected at 6 wk intervals over 15 mo. All 3 approaches showed that growth rate was approximately twice as fast on exposed as on sheltered shores (ANCOVA, p < 0.05 in all cases). Mean length for the first year of growth (averaged for the 3 methods) was 47 and 22 mm, respectively. There were, however, considerable differences among methods. Marking provided the lowest estimates of growth rates, and SLCA the highest. The mean mortality index (Z yr -1 ) was significantly higher and annual survival rate lower for exposed shores, indicating faster turnover under exposed conditions. Mean longevity on exposed shores (2.6 yr) was only one-third of estimated longevity on sheltered shores (6.7 yr). These findings show that degree of exposure has important interactions with growth and mortality of mussels on the low shore, and that these effects are independent of the effects of exposure on density. The findings also indicate strong bottom-up regulation of mussel populations.