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Grazing and suvival of larval zooplankton, Artemia salina and Acartia tonsa tonsa, , fed on wild and cultured populations of harmful algae
MA MARCOVAL; J PAN; CJ GOBLER
Ocean Shores, Washington, USA
Simposio; Fifth Symposium on Harmful Algae in the U.S.; 2009
NOAA/Center for Sponsored Coastal Ocean Research/Coastal Ocean Program NOAA/Northwest Fisheries Science Centers West Coast Center for Oceans and Human Health
Harmful algal blooms are a common feature during Spring-Summer in the coastal bays and lagoons of Long Island, New York, with blooms of Cochlodinium polykrikoides, Aureococcus anophagefferens, and Alexandrium fundyense occurring annually. Larval stages of zooplankton represent an important link between primary producers and higher-trophic-level consumers in marine food webs. We looked into the effects of harmful algae on nauplii of Artemia salina and copepodites of Acartia tonsa, in grazing experiments performed at different dilutions (0, 20, 50, 80 and 100%) of an initial concentration of the microalgae. The algal species used for experiments were the pelagophyte A. anophagefferens (strains CCMP 1984, 1850 and 1707) at initial concentrations between 800,000-106 cells ml-1, and the dinoflagellates Cochlodinium 1,500-2,000 cells ml-1, and Alexandrium 1,200-1,500 cells ml-1. The non-harmful cryptophyte Rhodomonas sp. was used as a control treatment (~1,500 cells ml-1). Experiments lasted for 24 h and zooplankton clearance rates (CR), survival and behavioral changes were determined. For the CCMP 1984 dilutions, CRs were greater for A. salina than A. tonsa, presenting the highest values at 50% dilution (80 and 25 µl ind h-1, respectively). The highest clearances in the CCMP 1707 treatments were at 80% dilution with values ~100 µl ind h-1 for both species. In the case of CCMP 1850, A. salina showed highest clearances at 20% dilutions (40 µl ind h-1), while A. tonsa showed negative clearances for all concentrations tested. The experiments with Cochlodinium showed similar results, with higher CRs for A. salina (ranging from 400 to 500 µl ind h-1), but with no significant differences between dilutions, while the highest values for A. tonsa were recorded at 100% initial concentration (~150 µl ind h-1) decreasing as the sample was further diluted. A. salina responded to the presence of Alexandrium with higher CRs for the 20% of the initial concentration (200 µl ind h-1). A. tonsa, however, showed no feeding, with negative values of CR. When fed Aureococcus and Cochlodinium, neither zooplankton species presented abnormalities in the functioning of appendages, and average survival was 90% for A. salina and 70% for A. tonsa. When fed Alexandrium at highest concentrations, survival for A. salina was 60% while survival for A. tonsa was 30%. Due to the elevated resistance and ability to feed on harmful algae displayed by Artemia salina, incubation experiments were conducted with nauplii of Artemia salina (at a concentration of 1 ind ml-1) feeding on the natural plankton community during blooms of Alexandrium sp. (Apr-May 2008) at initial concentrations of 200-600 cells ml-1, A. anophagefferens (May-Jul 2008) at 400,000-1.2 x106 cells ml-1 and Cochlodinium sp. (Aug-Sep 2008) at 300-2,000 cells ml-1. Experiments lasted for 24 h and behavioral changes, percent survival and grazing rates by A. salina were determined. Moreover, changes in the planktonic community were determined from preserved samples, through microscopic analysis, immuno-specific counting methods, and chlorophyll-a determinations. The highest filtration rates estimated for Artemia in this study (0.20 ìg chl h ind-1) coincided with high concentrations of Alexandrium sp. (674 cells ml-1), while the lowest clearance rates (0.05 ìg chl h ind-1) were recorded with low concentrations of this species (100 cells ml-1). Similar results were obtained regarding the decrease of chlorophyll-a fractions during blooms of the other harmful species studied. For example, after 24-h incubations with Aureoccocus bloom water, values of chl-a decreased from 25 to 5 ìg l-1. Conversely, during the bloom of Cochlodinium sp., the concentration of cells decreased with respect to the rest of the community; initial concentrations of 1,475 cells ml-1, decreased to 880 cells ml-1 at the end of the experiment. These results indicate that Artemia may not be a selective feeder, but that it consumes the most abundant species within the community. A survival of ~90% was estimated for Artemia feeding on blooms of Aureococcus and Cochlodinium, with the highest mortality rates (40%) occurring during the bloom of Alexandrium; the later being the only microalgal species of the three used in this study, known to produce a toxin.