C.5.2.4 Nutrient chemistry (PIs: Loder, Townsend).

Rationale: We hypothesize that some combination of the forms of dissolved nutrients and elemental ratios allow the growth of Alexandrium cells when other species of phytoplankton may be nutrient- (or light-) limited. This scenario implies some form of water mass "conditioning" and may explain the tight association between Alexandrium and the WMCC, which contains humic and other terrestrial materials. A problem with efforts to establish relationships between nutrients and cell densities may be that organic forms of nutrients have been neglected; our earlier work reveals that organic N in the WMCC exceeds inorganic forms by a factor of 2-5 (Loder, unpublished). The objective of the nutrient measurements in ECOHAB is thus to document the known and potentially important pools that are directly associated with Alexandrium or accessible to it by vertical migration or other processes.

Approach: Dissolved organic and inorganic nutrients will be measured at all stations in all WMCC and EMCC surveys. Some will also be analyzed for particulate N and P. During EMCC surveys, samples will be filtered, stored on ice, and analyzed at sea for dissolved inorganic nutrients and total dissolved N and P using a 5-channel Technicon AutoAnalyzer, using methods of Whitledge et al. (1986). Casco Bay samples will be filtered, frozen, and analyzed with a Lachat Autonalyzer using standard methods (Lachat, 1993; 1994a,b,c). Total dissolved N and P (TDN, TDP) will be determined following Valderrama (1981) using the Lachat Analyzer. Dissolved organic N and P are determined by difference. Particulate C, N and P will be determined at selected stations using standard methods. In situ chlorophyll fluorescence and light transmission will be measured at all hydrographic stations. Chlorophyll-a will be determined fluorometrically on discrete water samples.