Investigations into the seasonal deep chlorophyll maximum in the western North Atlantic, and its possible significande to regional food chain relationships

Abstract

In many marine environments accumulations of chlorophyll have been reported to occur at or below depths to which 1% of ambient light penetrates. The phenomenon has been called the Deep Chlorophyll Maximum (DCM). On occasion zooplankton have been observed to be suggestively associated with a DCM. In order to determine, to what extent and under what circumstances, the DCM represents a significant food resource, data were obtained from vertically stratified net tows (both 0.333 μm and 0.067 μm mesh) and water bottle casts taken on eight cruises in the western North Atlantic between November 1973 and August 1976. Parameters measured included: zooplankton biomass, zooplankton functional group abundance, phytoplankton species abundance, chlorophyll concentration, ATP concentration, particulate nitrogen concentration, 14C fixation, biological macro-nutrients (N03, NO2, NH3, P04, Si(OH)4), oxygen concentration, temperature, and salinity. Parameters were measured as concommitanty as possible. Sampling was conducted in the Sargasso Sea, in Gulf Stream cold core rings, and in the Slope Water. Results obtained bear upon three major ecological problems: (a) the evolution of the biological community in a Gulf Stream cold core ring; (b) the sense in which the Gulf Stream represents an ecological discontinuity; and (c) the significance of the DCM as a locus for trophic activity. Zooplankton biomass in the upper 800 m of four Gulf Stream cold core rings significantly exceeded that in the Northern Sargasso Sea. The center of its vertical distribution was uniquely deep. Such a distribution may result in reduced ecological efficiency and increase the flux of organic matter to the deep sea. The phytoplankton assemblage of a cold core ring was significantly different from that of both the Slope Water and the Northern Sargasso Sea many months after ring formation. Certain species appeared to capitalize on some aspect of the ring environment and were especially numerous in ring samples. Due to the composition, distribution, and variability of its characteristic phytoplankton the Slope Water represented a herbivore habitat very different from that in either the Northern Sargasso Sea or a six-month-old cold core ring. Under highly stratified conditions the preceding contrast was maximal. No common species was found only on one of the other side of the Gulf Stream, yet the species could be sorted into groups that had maximal abundances either in the Slope Water or the Northern Sargasso Sea. These groups appeared to differ in their responsiveness to nutrient concentration variation. The DCM in diverse environments appeared to be an essentially identical phenomenon. The DCM accumulated phytoplankton cells (and possibly other organic particulates) sinking from above. Phytoplankton growth occurred as DCM depths despite low light levels. Various microbial processes appeared to be enhanced at DCM depths. As a consequence the DCM signalled a depth zone which, under stratified conditions, was a significant food resource especially since mixed-layer food was scarce. Concentrations of zooplankton biomass at the DCM and the vertical distributions of zooplankton functional groups indicated the DCM in the western North Atlantic was a locus of particularly intense trophic activity. The depth interval of the DCM had more total biomass and more microplankton biomass than above and below. Further, at DCM depths, the abundance of particular zooplankton functional groups appeared to reflect the size of the dominant phytoplankton. Not only presumed herbivores but a purely carnivorous group, the chaetognaths, on some occasions aggregated at DCM depths.