Population dynamics and spatial distribution of phycotoxic microalgae associated with shellfish aquaculture sites in Nova Scotia

Abstract

Summary form only given. Coastal shellfish aquaculture sites are subject to seasonal blooms of toxic microalgae, which may result in restrictions on shellfish harvest due to human health risk. In southeastern Nova Scotia, such toxic episodes are uncommon, although significant numbers of putatively toxigenic phytoplankton are often found in the water column. In most coastal regions where diarrhetic shellfish poisoning (DSP) incidents have occurred, the events have been associated with blooms of species of the planktonic dinoflagellate Dinophysis. However, despite repeated attempts over several years, the authors have been unable to link the presence of DSP toxicity in Nova Scotian shellfish to the timing and magnitude of seasonal blooms of Dinophysis spp., even using sophisticated analytical techniques for toxin detection in size-fractionated planktonic material. Accordingly, the authors are now focusing on the epibenthic/epiphytic microalgal community associated with the suspended culture of bivalve shellfish in Mahone Bay, N.S., specifically blue mussels (Mytilus edulis). Loosely aggregated material (termed slub) on shellfish lines was found to contain low levels of DSP toxins. Significant numbers of cells of a toxigenic epibenthic dinoflagellate, Prorocentrum lima were associated with the epiphytic macroalga Pilayella littoralis, a common fouling organism on suspended shellfish lines. Unialgal isolates of P. lima from this material were confirmed to produce DSP toxins in culture. Spirolides, a new class of biologically-active marine compounds, have also been identified within the digestive tissues of blue mussels and sea scallops from the east coast of Nova Scotia, including sites in Mahone Bay. The spatio-temporal distribution and seasonal occurrence (typically early May to July) of spirolides in shellfish strongly suggested a planktonic origin, and these compounds have been traced to specific size-fractions (predominately 20-50 /spl mu/m) of plankton from the upper water column. An unusual group of pigmented spherical cells (golden balls), with taxonomic affinities to certain gonyaulacoid dinoflagellates, are dominant in these fractions. To establish the role of attached and suspended particulate material surrounding shellfish aquaculture installations on phycotoxin transfer and feeding mechanisms, data were collected on standard oceanographic parameters (chlorophyll, inorganic nutrients, seston, POM/PIM, plankton composition and abundance, water column stratification). Preliminary studies with a high-resolution underwater photo-imaging system provided an extended time-series (48 h) of the vertical distribution of attached and suspended particulates (marine snow). The particle field immediately adjacent to the shellfish lines was clearly distinct from that of a nearby control site (30 m distant) and the phytoplankton composition was unrepresentative. This indicates a potentially serious bias in the application of conventional techniques for water column monitoring of toxigenic phytoplankton in nearshore coastal zones.