Biological Removal of Particles During Chalk-Ex: Results of Shipboard Bottle Experiments

As part of the Chalk-Ex program, 13 tons of powdered Cretaceous chalk were dispersed into a patch of about 1.5 km2 in the NW Atlantic Ocean in November 2001. The chalk patch was then followed for several days while physical properties of the water were measured. A series of related abstracts details those results. To estimate the possible role of grazing zooplankton in removal of the chalk, we conducted five bottle incubations on deck while the patch was being followed. Experiments consisted of four treatments. For all treatments, one-liter polycarbonate bottles were filled with water pre-screened through 200 um mesh by reverse flow. Three of the treatments received added mesozooplankton in successively greater multiples of the natural abundance, up to c. 32x.. The experiments were designed to look for evidence of microzooplankton grazing (removal of chalk-sized particles in the <200 um treatment) and either top-down control by mesozooplankton (grazing on chalk-sized particles declines as mesozooplankton concentration increases) or direct mesozooplankton grazing (grazing on chalk-sized particles increases with mesozooplankton concentration). Abundance of chalk-sized particles (1.3-3.5 um) was measured with an Elzone particle counter at initial and final (c. 20h) experimental times. We also had formalin-killed controls, but they showed evidence of particle production during the experiments, most likely due to mineral precipitation of the preservative's buffer, and were thus difficult to interpret. Net per capita rates of change for chalk-sized particles ranged from -0.89 to +0.47 d-1 in the <200 um treatment during the five experiments. Three of the five experiments showed evidence of top-down control on microzooplankton grazing. Because the chalk size-fraction also contained bacterioplankton, which presumably was growing during the incubations, any net losses would provide minimal estimates of potential grazing on chalk. For several experiments, chalk particles were abundant enough to count directly using a polarized light microscope. These observations indicated real net loss of chalk particles in the bottles. Because processes other than grazing might have contributed to loss of chalk particles in the bottles (e.g. dissolution or aggregation; see abstract by Dam et al.), we plan future work on possible chalk ingestion by protists using laboratory cultures.