Assessment of the particulate food supply available for mussel (Mytilus spp.) farming in a semi-enclosed, northern inlet

Temporal variability in the quantity, organic content, and phytoplankton co mposition of the particulate food supply available to a cultured mussel pop ulation was assessed for a 3-year period in a small inlet of Notre Dame Bay , Newfoundland, Canada. The study site had a restricted flushing rate estim ated at 1-2.75 times wk(-1) for a complete water exchange. The quantity of both total (TPM) and organic (POM) seston varied temporally from 0.7-23.7 m g l(-1) and 0.05-1.97 mg l(-1) respectively during the 3-year sampling peri od. TPM typically remained relatively high (> 10 mg l(-1)) through the wint er and spring period. Most of the seasonal variation in total seston was du e to seasonal variability in the PIM component. Both PIM and POM concentrat ions were seasonally lowest during summer. The organic fraction of the sest on (POM/TPM ratio) was seasonally low in winter and increased steadily thro ugh spring and summer to reach its maximum in the autumn. The living phytop lankton component of the seston was typically dominated, both numerically a nd in biomass, by a variety of diatom and autotrophic nanoflagellate specie s in the 2-20-mum diameter size range. Discrete diatom population blooms oc curred in the autumn of all three years and largely consisted of a single s pecies, Skeletonema costatum. Phytoplankton:detritus ratios were significan tly lower during winter. Total phytoplankton biomass levels were seasonally low during winter and summer and were associated with seasonal variation i n diatom biomass. We conducted modelling simulations of relationships among seston organic food levels, their temporal variability, tidal flushing rat es, cultured mussel biomass and production indices, and estimates of mussel maintenance ration requirements to predict the adequacy of northern inlets to sustain commercial-scale mussel farm development. We conclude from thes e simulations that small, semi-enclosed, northern inlets likely frequently experience periods when naturally occurring organic seston levels are insuf ficient to meet basic mussel stock maintenance ration requirements when suc h inlets are stocked to commercial-scale densities. We infer that periods o f food-limited mussel growth and/or production are likely to occur at these high mussel population levels, particularly during summer. Qualitatively p oor (low POM:TPM ratio; low phytoplankton:detritus ratio) food supply durin g winter may also be a further limiting factor. The implications of our con clusions in relation to aquaculture industry development and environmental management are discussed. (C) 2001 Academic Press.