Spatial distribution and abundance of larval and juvenile fish, chlorophyll and macrozooplankton around the Mississippi River discharge plume, and the role of the plume in fish recruitment

Abstract

In September 1986, we collected neuston (1 x 2 m, 0.947 mm mesh) and surface chlorophyll a samples and hydrographic data at 46 stations around the discharge plume of the Mississippi River. Transects were positioned so that the 3 water masses in the plume area -plume water, Gulf of Mexico shelf water and frontal water (a mixture of the former 2) -were sampled. The plume was represented by a shallow lens of water < 3 4 '10 salinity and < 29 "C resting atop warmer (> 29 "C) and more saline (> 34 %) Gulf of Mexico shelf water. Strong turbidity fronts with a scale of 50 to 100 m form, relax and reform approximately at tidal frequencies within the frontal region that has a larger scale of 6 to 8 km. Total ichthyoplankton catch per tow, individual surface chlorophyll a values and macrozooplankton displacement volumes were all significantly greater in frontal waters than adjacent Gulf of Mexico shelf or plume waters Hydrodynamic convergence at the continually forming and relaxing turbidity fronts most likely accounts for concentrated neustonic ichthyoplankton, and at least partially for high macrozooplankton values as well, in frontal waters. Elevated macrozooplankton displacement volumes in frontal waters may also result from higher rates of proliferation of macrozooplankton biomass. High primary production in frontal water is probably due to the mixing of nutrientrich, but turbid, plume water (where photosynthesis is light limited) with clear, but nutrient-poor, Gulf of Mexico shelf water (where photosynthesis is nutrient limited) creating favorable phytoplankton growth conditions. Concentrations of ichthyoplankton and zooplankton offer rich trophic resources that some species utilize to gain superior growth. Faster growth will lead to increased survival and recruitment (because larvae pass through the period of greatest vulnerability to predation by gapelimited predators more quickly) if larval growth is increased disportionately to larval mortality from predation.

Selective foraging within estuarine plume fronts by an inshore resident seabird

“…The presence of penguins in productive waters is in line with several studies that found that seabirds forage in areas of elevated levels of primary productivity (Weimerskirch et al, 2004;Ainley et al, 2005;Suryan et al, 2012). Areas with high Chl-a content are associated with sustained primary productivity and are therefore more likely to attract and aggregate planktivores that in turn provide predictable food sources for planktivorous fish and their predators (Grimes and Finucane, 1991;Ressler et al, 2005;Scales et al, 2014).…”

“…Entrainment of phytoplankton and zooplankton attract foraging fish, making estuarine plume fronts important nearshore foraging features for marine predators, particularly seabirds (Grimes and Kingsford, 1996;Skov and Prins, 2001;Zamon et al, 2014). However, the dynamic nature of these water masses result in large physical and physico-chemical fluctuations (e.g., temperature, salinity and dissolved oxygen), which influence local prey distribution, composition and biomass (Grimes and Finucane, 1991;Wagner and Austin, 1999). This variability has subsequent effects on the distribution of marine predators, whose at-sea distribution is mostly controlled by the occurrence of their prey as well as their physiological and breeding energetic constraints (Wakefield et al, 2009;Zamon et al, 2014).…”

“…Within these areas, mixing, and nutrient retention enhance primary productivity, which in turn attract and aggregate zooplankton (Grimes and Finucane, 1991). Entrainment of phytoplankton and zooplankton attract foraging fish, making estuarine plume fronts important nearshore foraging features for marine predators, particularly seabirds (Grimes and Kingsford, 1996;Skov and Prins, 2001;Zamon et al, 2014).…”

Phytoplankton Biomass and Production in Subtropical Hong Kong Waters: Influence of the Pearl River Outflow

“…An unusually large summer bloom (up to 73 μg chl L −1 ) was observed in this region (Stn S12), accompanied by a drawdown of PO 4 to limiting concentrations (Xu et al 2009). The frontal region of the plume has also been found to have high chl a concentrations relative to inshore and offshore waters in other coastal waters such as Chesapeake Bay (Breitburg 1990;Harding 1994) and the Mississippi River plume (Grimes and Finucane 1991).…”

Fish Resources of the Gulf of Mexico

“…There is large spatiotemporal variability in oceanographic conditions, with the Gulf of Mexico influenced greatly by inflows and discharges from rivers and other land-based sources, including the Mississippi River, and by large-scale oceanographic features, such as the Loop Current and associated core eddies of different thermal conditions (Govoni and Grimes 1992; Sturges and Leben 2000). Combined, these factors result in large spatiotemporal variability in physicochemical conditions, causing primary production to vary markedly within and across areas of different oceanographic conditions in the Gulf of Mexico (Grimes and Finucane 1991;Biggs 1992). Physical-chemical variability affects the distribution, growth, and mortality of pelagic larvae of many fish species in the Gulf of Mexico (Govoni et al 1989;DeVries et al 1990;Lang et al 1994).…”