Colonial Aggregates: Effects of Spatial Position on Zebra Mussel Responses to Vertical Gradients in Interstitial Water Quality

Abstract

Vertical gradients in interstitial water quality may develop within densely organized assemblages of sessile aquatic organisms. These gradients may compromise the survival of individuals. We examined whether a vertical gradient of interstitial water chemistry (NO₃-N, NH₄-N, and dissolved oxygen [DO]) would develop within dense zebra mussel (Dreissena polymorpha) colonies in a laboratory flume (flow rate ∼1 cm/s). Over a 4-h duration, we found that NO₃-N concentrations increased, DO decreased, and NH₄-N concentrations remained the same from the surface to the base of 6-cm thick zebra mussel colonies. These results were supported by trends found in natural Lake Michigan zebra mussel colonies at 4 to 6 m depths, where NO₃-N concentrations at the base of colonies measured 162% of NO₃-N concentrations in open water above the colonies. We also examined how vertical water-quality gradients influenced zebra mussel movement and mortality by tracking the vertical position and survivorship of individual zebra mussels in colonies. Substantial movement out of the base of the colony occurred after 7 and 30 d of incubation. After 30 d, 69% of smaller-sized mussels (≤6 mm) moved upward within the colony from the base, in contrast to 0% movement of larger mussels (>20 mm), whose motility may have been impeded. After 30 d, mortality of all size classes significantly increased, with >50% mortality occurring in the bottom layer. Our studies suggest that dense colonies produce vertical interstitial water-quality gradients at low flow, and that movement out of the base of the colony by smaller mussels may be an important mechanism for survival in dense colonies.