Saldanha Bay, South Africa III: new production and carrying capacity for bivalve aquaculture

Abstract

Measurements of NH4, NO3, urea and HCO3 uptake using 15N and 13C stable isotope tracers were undertaken in Saldanha Bay, South Africa, between January 2012 and January 2013. These studies provide the first direct measurements of N utilisation by the plankton in the bay. Primary production in the bay is driven predominantly by the advection of nutrients from the neighbouring shelf environment during upwelling events, with terrestrial and other sources providing minor inputs. New production (NO3-based) was calculated from the f-ratio and total primary production and was used to provide estimates of potential carrying capacity for bivalve culture. Despite the apparent light limitation of NO3 uptake in the winter, the availability of NO3 appeared to exert the major influence on new production throughout the year. In addition, new production was modulated by NH4 availability as shown by the suppression of NO3 uptake by concentrations higher than 1−1.5 mmol m−3. The estimated areal new production of 0.60 g C m−2 d−1 yielded a bay-wide annual estimate of 9 811 t C ha−1 y−1, slightly higher than previous calculations based on physical models. It is estimated that the total annual production of mussels and oysters, respectively, for a 1 000-ha cultivation area is approximately 40 000–53 000 t y−1 (mainly Mytilus galloprovincialis) and 4 600–6 000 t y−1 (Crassotrea gigas). The combined total production figures constitute only 24–31% of the surplus new production. A combined harvestable carrying capacity of 74 000–82 000 t y−1 can be calculated from this surplus. However, from a management and ecological perspective, bivalve culture should be limited to well below this theoretical maximum. Even with this constraint, there appears to be considerable scope for expansion of bivalve farming over the modest, present levels with little jeopardy to ecological integrity.