Florida Bay, a shallow, seagrass-dominated bay in southern Florida, USA, receives significant nutrient inputs and has experienced seagrass losses and microalgal blooms within the last several decades. Inorganic nutrient inputs have been well characterized, but the role of organic nutrients, specifically of dissolved organic nitrogen (DON) and organic phosphorus (DOP), in supporting microbial processes in the bay is unknown. In this study various techniques were used to assess the importance of these nutrients along a transect in Florida Bay when a cyanobacterial bloom occurred in the central region in November 2002. These techniques included measurements of ambient particulate and dissolved nutrients, enzyme (urease and alkaline phosphatase) activities, and experiments to determine rates of 15 N uptake (nitrate, ammonium, urea, and amino acids over a period of 0.5 h) and long-term (48 h) changes in microbial biomass and 15 N natural abundance in enrichment bioassays. The cyanobacterial bloom in central Florida Bay was associated with the highest concentrations of DON and DOP, whereas the microflagellate-and diatom-dominated eastern bay region was associated with the highest concentrations of inorganic nutrients. The zeaxanthin:chlorophyll a ratio (an indicator of the relative contribution of cyanobacteria to phytoplankton biomass) was positively correlated with the rate of uptake of urea, and negatively correlated with the rate of uptake of inorganic nitrogen. The opposite pattern was observed for the fucoxanthin:chlorophyll a ratio (indicative of relative diatom biomass) and the peridinin:chlorophyll a ratio (indicative of relative photosynthetic dinoflagellate biomass), suggesting that different algal groups were using different N substrates. Biomass responses in the bioassay experiments showed that phytoplankton (as chlorophyll a) responded to DON additions in the western region and to DOP additions in the eastern region, but heterotrophic bacteria, in contrast, responded to DOP additions in the west and DON additions in the east. These findings thus demonstrate the potential for different sources of N, including DON, to stimulate different components of the algal community, and for the phytoplankton and bacteria to respond differently to N and P.KEY WORDS: Florida Bay · Cyanobacteria · Organic N · Organic P · Nutrient limitation · Urease · Alkaline phosphatase · Bioassay · Nutrient uptake rates Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 280: [73][74][75][76][77][78][79][80][81][82][83] 2004 changes. Since the onset of industrialization in the 1880s, the health of the Florida Bay ecosystem has been negatively impacted on both decadal (e.g. increasing eutrophication) and centurial (e.g. changes in land use and water management practices within southern Florida) time-scales (Fourqurean & Robblee 1999). These anthropogenic changes have led to a significant alteration in freshwater flow patterns within the Everglades, causing declines in seagrass d...

During May 2003, the dry season, the southwest Florida shelf was surveyed for nutrient concentrations and phytoplankton community composition concurrently with plankton nutritional and physiological status (tracer techniques, enzyme assay, and biomass response bioassays). Inorganic nitrogen concentrations were low throughout the region, while dissolved organic nitrogen (DON) was elevated (.30 mmol N L 21 ) nearshore. Conversely, PO 3{ 4 and dissolved organic phosphorus concentrations were high only in northern coastal areas adjacent to estuarine outflows. Nitrogen : phosphorus (N : P) ratios in the particulate material; gradients in NO { 3 and urea uptake rates; urease and alkaline phosphatase activities; and bioassay responses were indicative of a strong gradient from N to P limitation of plankton biomass from north to south. Nitrogen limitation was apparent in the northern region (N : P particulate , 8, Tampa to Sanibel), where PO 3{ 4 and DON inputs dominated; balanced nutrient conditions were evident in the mid-region (N : P particulate 5 8-24, Sanibel to Shark River); and P limitation was evident south of the Shark River (N : P particulate . 24), where inorganic N input was greatest. Phytoplankton community composition varied along the same gradient, from a cyanobacteria and dinoflagellate community in the north, to a cyanobacteria-dominated community in the mid-region, and to a diatom community in the southern region. Percentages of NO { 3 uptake and diatoms in the total phytoplankton community were positively related, as were percentages of urea uptake and cyanobacteria. Inorganic and organic N and P fractions in the nearshore shelf region reflect longitudinal gradients in regional watershed characteristics, and their relative availability thus appeared to control both phytoplankton community composition and its physiological status.

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This study presents a comparison of two existing methods for the determination of urea concentration in seawater. These methods are referred to here as the enzymatic method, which is based on the use of the enzyme urease, and the direct method, which is based on the reaction of urea with diacetylmonoxime. A room temperature modification of the direct method was adapted for a single reagent and both the enzymatic and the direct method were tested in artificially prepared solutions and in natural samples from estuaries and shelf waters. We were particularly interested in the effects of salinity and humic acids on the accuracy of both methods. The effect of humic acids was negligible. In contrast, salinity ~34 caused a 15% to 40% underestimation in the urea concentrations measured by the enzymatic method and the degree of underestimation varied among enzyme batches. Urea concentrations corrected for the salt effect should, however, be considered estimations, as other factors also interfered with the enzymatic method in natural samples. The direct method as modified in this study presented a low detection limit (0.04 µM urea‐N) and high precision (standard deviation: 0.02 µM urea‐N; coefficient of variation: 1.1%) comparable to those of the enzymatic method. The direct method was more accurate and less salinity dependent than the enzymatic method. As urea levels could have been underestimated by the enzymatic method, our findings support previous conclusions regarding the important role of urea in the nitrogen cycle and its link with some harmful algal bloom phenomena.