The Effect of the Sinking Spring Diatom Bloom on Digestive Processes of the Cold-Water Protobranch Yoldia hyperborea

Periodic sampling from April 1997 to December 1998 of a population of the deposit-feeding protobranch Yoldia hyperborea from the deep depositional zone (250 m) of Conception Bay, Newfoundland, indicated that this species was exposed to subzero temperatures (-0.63°C) throughout the year. However, the standing stock of phytoplankton in the photic zone showed strong seasonal fluctuations with primary peaks always occurring in April (4.26 and 5.09 μg L-1 chlorophyll a in 1997 and 1998, respectively). In addition, sediment chlorophyll a started to increase (from a low of ∼6 ng mg-1 dry weight of sediment) as the sinking of a smaller, secondary bloom, reached the bottom in the first 2 months of 1997 and 1998, although higher concentrations were reached by mid-April and May 1997 (21.32 ng mg-1 sediment) and remained relatively high until February 1998. An increase of digestive cell height and protein content occurred as soon as sinking organic material from the spring bloom reached the benthic zone, suggesting storage of metabolic energy during spring and summer. Sharp increases in digestive enzyme activity occurred primarily in early spring of each year, coinciding with the timing of the primary spring bloom fallout, suggesting activation of the formerly depressed digestive system after a prolonged period of low food availability (i.e., late summer to autumn). Results suggest that Y. hyperborea strongly depends on the input of fresh algal material despite the high availability of organic matter in the sediment.