How do Riverine Plumes of Different Sizes Influence Fish Larvae: do they Enhance Recruitment?

Abstract

A review of the literature suggests that river discharge plumes strongly influence fish larvae and may play a significant role in the recruitment of local fishes. Some rivers drain large land masses to discharge shallow, turbid and nutrient-rich plumes that interact with ocean currents as well as with local oceanography and meteorology; these plumes may extend hundreds of kilometres offshore and alongshore. The frontal, or mixing, zone between plume and ocean waters is characterized by strong physical and biological processes. Physical dynamics, e.g. hydrodynamic convergence, and abundant nutrients (both river derived and upwelled) in the vicinity of discharge plumes often generate large stocks of phytoplankton, zooplankton and fish larvae, as well as high rates of primary and secondary production. Physical dynamics not only act to accumulate (and probably retain) biomass in frontal waters, but also transport organisms onshore, offshore and along the frontal boundary. The mechanisms through which river plumes may influence recruitment are not clear. In considering the potential effects of scale of river discharge on recruitment, three alternative hypotheses are discussed. The short-food-chain hypothesis states that recruitment will be enhanced in the vicinity of river plumes because fish larvae experience superior feeding conditions, grow faster and thus experience a shorter stage duration and survive better. The total-larval-production hypothesis is that trophic conditions support such high total production of fish larvae that specific dynamics of growth and mortality are not relevant. The third hypothesis is that plumes facilitate the retention of fish larvae within a limited area, and it is the physical retention rather than production that explains the variation in recruitment. If one or a combination of these hypotheses explains the influence of river plumes on recruitment, then the greatest potential to affect fish recruitment may be possessed by large mid-latitude rivers carrying high suspended-sediment and nutrient loads that discharge into shelf waters to create well defined plumes where primary and secondary production are high.