Abstract. Relative to inorganic nitrogen, concentrations of dissolved organic nitrogen (DON) are often high, even in regions believed to be nitrogen-limited. The persistence of these high concentrations led to the view that the DON pool was largely refractory and therefore unimportant to plankton nutrition. Any DON that was utilized was believed to fuel bacterial production. More recent work, however, indicates that fluxes into and out of the DON pool can be large, and that the constancy in concentration is a function of tightly coupled production and consumption processes. Evidence is also accumulating which indicates that phytoplankton, including a number of harmful species, may obtain a substantial part of their nitrogen nutrition from organic compounds. Ongoing research includes ways to discriminate between autotrophic and heterotrophic utilization, as well as a number of mechanisms, such as cell surface enzymes and photochemical decomposition, that could facilitate phytoplankton use of DON components.

In situ analysis of phytoplankton community structure was determined at five stations along the Texas Gulf coast using two instruments, the Fluoroprobe and FlowCAM. Results were compared with traditional methods to determine community structure (pigment analysis and microscopy). Diatoms and small nanoplankton (most likely haptophytes) dominated the phytoplankton community at all stations. Estimated chl concentrations for diatoms+dinoflagellates obtained via the Fluoroprobe were not significantly different for three of the five stations sampled when compared with HPLC‐chemical taxonomy analysis, whereas the concentrations of green algal and cryptophyte chl were overestimated. The FlowCAM estimates of overall nanoplankton and microplankton cell abundance were not significantly different when compared with epifluorescence microscopy, and recorded images of phytoplankton cells provided a representative population of the phytoplankton community at each station. The Fluoroprobe and FlowCAM, when used in tandem, are potentially capable of determining the general characteristics of phytoplankton community structure in situ and could be an important addition to biological observing systems in the coastal ocean.

Humic substances are a collection of colored organic acids characterized by high molecular weight and low nitrogen (N) content that are thought to be biologically recalcitrant. We examined a suite of nonaxenic estuarine phytoplankton isolates to determine their ability to take up 15 N-labeled humic substances formed in the laboratory and supplied as the sole N source. All 17 estuarine and coastal strains took up the added humic N, but the one polar isolate did not. Two of the coastal isolates (Heterosigma akashiwo and Fibrocapsa japonica) could take up the humic N in nonaxenic culture but not in axenic culture, suggesting that bacterial remineralization played a role in making humic N accessible to these species. The ability of nonaxenic phytoplankton isolates to use humics of different ages (1 week to 1 yr old) was tested using three strains capable of taking up humic N at high rates. Younger, fresher humics were taken up by the phytoplankton strains at higher rates than older, more fulvic-like compounds, and at rates higher than inorganic N uptake run in parallel. Time-course results indicate that while uptake of the inorganic N forms was sustained, high rates of humic N uptake declined after the first few hours of incubation. Additional humic substances were labeled with both 15 N and 13 C, and the relative incorporation of N versus carbon (C) was used to infer potential uptake mechanisms. None of the isolates took up humic C, suggesting that uptake of the humic N followed breakdown of the humic molecule by bacteria or via extracellular enzyme cleavage of humic N. Regardless of the mode of uptake, the observation that humic N can be rapidly used by phytoplankton suggests that the importance of humic N as a source of phytoplankton N nutrition should be reevaluated.

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