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Trophic Interactions between Anadromous Juvenile Alewife (Alosa pseudoharengus) and Cyanobacterial Populations in a Shallow Mesotrophic Pond

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Document pages: 26 pages

Abstract: Alosa pseudoharengus is an anadromous fish that migrates from marine tofreshwaters to spawn. The early larval and juvenile forms are known to beplanktivorous, where heavy feeding upon their preferred food source of largecrustacean zooplankton often results in changes to composition and sizestructure within this trophic guild which in turn can result in shifts withinthe trophic spectrum and a classic trophic cascade. In this study of Lower MillPond, Brewster MA, we evaluated the feeding strategy of juvenile Alosa to determine whether juvenile alewife switches to feeding largely on cyanobacteria andwhether cyanotoxins microcystin (MC) and β-methlyamino-L-alanine (BMAA)bioaccumulate in their muscle tissue. Within 15 - 30 daysof their estimated spawning date, overexploitation of crustacean zooplanktonresulted in a shift from planktivory to benthic detritivory for the majority oftheir life history, although this did not reduce their condition based onweight-length relationships (Log Wwt. = -5.503 + (3.101 × LogLength). Mean MC (0.003 μg·g-1 dwt) and BMAA (4.49 μg·g-1 dwt) concentrations in the muscle tissue of out-migrating juveniles werepresumably derived from benthic subsidies, exporting freshwater cyanotoxins andcreating a potential transfer to consumer of 0.0012 μg MC and 1.85 μg BMAA.Biodilution of MC and biomagnification of BMAA were observed. Depletion of thecrustacean biomass by >95 resulted in an increase in the rotifer biomass,where Log crustacean (μg·L-1 dwt) = - 5.642 - (7.976 × Log rotifer (μg·L-1 dwt), and anincrease in the amount of potentially edible  - 0.676, p = 0.046). A secondarycascade appears to have been maintained via invertebrate planktivory by Chaoborus spp.; however for a period of time edible cyanobacteria growth exceeded grazingpressure, resulting in a bloom of edible cyanobacteria. Continued grazingresulted in a shift to larger, inedible cyanobacterial communities where lateseason (October) surface accumulations were observed. The mass occurrence ofjuvenile Alosa pseudoharengus appears to be coupled to thesequential increases of cyanobacterial biomass via its influence on the trophicspectrum. Overall, the rotifer biomass (μg·L-1) was positivelycorrelated with MC (pg·mL-1) (r(8) = 0.577, p = 0.104), and negatively correlated with BMAA (μg·L-1)(r(8) = - 0.388, p = 0.373) in the edible cyanobacterial fraction of the water column, althoughneither of these were significant.

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