Molecular-level variations in particulate organic matter subclasses along the Mid-Atlantic Bight

Abstract

Abstract A significant portion of oceanic primary productivity occurs on continental margins. As the fate of this primary production has not been clearly determined, continental margins have been the focus of increased research over the past two decades. In the present study, molecular-level characteristics of particulate organic matter (POM) subclasses from the Mid-Atlantic Bight (MAB) were explored in an attempt to further our understanding of organic matter cycling in productive continental shelf and slope waters. Small-particle POM (2→53 μm), large-particle POM (>53 μm), “phytoplankton” (2→53 μm) and “detritus” (2→53 μm) were isolated from surface waters collected from different regimes (mid-shelf, shelf-edge, and slope) along the MAB in March 1996. Broad-band molecular-level variations within these subclasses were explored using direct temperature-resolved mass spectrometry (DT–MS) and multivariate analysis techniques. Correlations between molecular-level compositional differences and external environmental and biological variables such as salinity, temperature, and chlorophyll a concentrations were also examined. Both large-particle and small-particle POM exhibited molecular-level variations related to regime. Large-particle POM from the continental shelf exhibited chemical characteristics consistent with a greater zooplankton biomass component, as indicated by an enhanced chitin, protein, and cholesterol signature. In contrast, large-particle POM from slope waters appeared to contain a greater phytodetrital component, as indicated by enrichment in molecular-level characteristics similar to those of phytoplankton. A higher relative proportion of phytoplankton biomass was present in small-particle POM from the shelf-edge than from the mid-shelf or the slope. In addition, there was also an alongshelf (roughly north–south) trend in the composition of 2→53 μm POM, with samples farther south exhibiting enrichment in polysaccharides, fatty acids, chlorophyll, and diglycerides (the latter may result from triglycerides, phospholipids, or diglycerides within the samples). The molecular-level composition of “phytoplankton” also appeared to vary as a function of location, while the composition of “detritus” illustrated no easily interpretable variations with either location or external variables.