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

Abstract

Alosa pseudoharengus is an anadromous fish that migrates from marine to freshwaters to spawn. The early larval and juvenile forms are known to be planktivorous, where heavy feeding upon their preferred food source of large crustacean zooplankton often results in changes to composition and size structure within this trophic guild which in turn can result in shifts within the trophic spectrum and a classic trophic cascade. In this study of Lower Mill Pond, Brewster MA, we evaluated the feeding strategy of juvenile Alosa to determine whether juvenile alewife switch es to feeding largely on cyanobacteria and whether cyanotoxins microcystin (MC) and β -methlyamino-L-alanine (BMAA) bioaccumulate in their muscle tissue. Within 15 - 30 days of their estimated spawning date, overexploitation of crustacean zooplankton resulted in a shift from planktivory to benthic detritivory for the majority of their life history, although this did not reduce their condition based on weight-length relationships (Log Wwt. = - 5.503 + (3.101 × Log Length). 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 were presumably derived from benthic subsidies, exporting freshwater cyanotoxins and creating 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 the crustacean 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 an increase in the amount of potentially edible <50 μm cyanobacterial biomass (r(8) = - 0.676, p = 0.046). A secondary cascade appears to have been maintained via invertebrate planktivory by Chaoborus spp. ; however for a period of time edible cyanobacteria growth exceeded grazing pressure, resulting in a bloom of edible cyanobacteria. Continued grazing resulted in a shift to larger, inedible cyanobacterial communities where late season (October) surface accumulations were observed. The mass occurrence of juvenile Alosa pseudoharengus appears to be coupled to the sequential increases of cyanobacterial biomass via its influence on the trophic spectrum. Overall, the rotifer biomass (μg · L -1 ) was positively correlated 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, although neither of these were significant.