Winter storms: Sequential sediment traps record Daphnia ephippial production, resuspension, and sediment interactions

Abstract

For species that do not over-winter, an essential part of the life cycle is the production of diapausing eggs. We use sequential sediment traps in southern Lake Michigan (1997-2000) to capture settling Daphnia ephippia and sediments, characterizing ephippial production and sediment interactions during hazardous conditions. Each year, there was an enormous pulse of D. mendotae ephippia (ca. 7.2 × 1013 ephippia, 1.2 × 1014 diapausing eggs) that coincided with autumn population decline. Most ephippia settled through the water column, although a few were captured at the water surface and blown shoreward. The duration and amplitude of the autumn ephippial fluxes were similar among years. Ephippial production was positively correlated with water column depth, a consequence of Daphnia spatial abundance. In contrast, resuspension of ephippia was inversely related to water column depth and spatially complex, influenced by waves, coastal currents, and offshore gyre circulation. Large winter storms created nearshore sediment plumes, could resuspend vast numbers of ephippia (e.g., 10 March 1998 storm; 1.9 × 1012 ephippia and 3.1 × 1012 diapausing eggs), and were important in the formation of "egg banks." Almost all newly produced and resuspended diapausing eggs came from D. mendotae, with very few from two other species (D. retrocurva, D. dentifera) that dominated waters 12 yr ago. These observations suggest a relatively short relaxation time for species cycling out of egg banks in Lake Michigan (<10 yr), due in part to (1) differential resuspension of unconsolidated versus consolidated sediments and (2) the spatially restricted nature of "high-sedimentation" zones. Our study is the first use of sequential sediment traps to document diapause egg production.