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Abstract: Nutrient loadings to urbanized estuaries have increased over the past decades in response to population growth and upgrading to secondary sewage treatment. Evidence from the San Francisco Estuary (SFE) indicates that increased ammonium (NH4) loads have resulted in reduced primary production, a counter-intuitive finding; the NH4 paradox. Phytoplankton uptake of nitrate (NO3), the largest pool of dissolved inorganic nitrogen, is necessary for blooms to occur in SFE. The relatively small pool of ambient NH4, by itself insufficient to support a bloom, prevents access to NO3 and bloom development. This has contributed to the current rarity of spring phytoplankton blooms in the northern SFE (Suisun Bay), in spite of high inorganic nutrient concentrations, improved water transparency and seasonally low biomass of bivalve grazers. The lack of blooms has likely contributed to deleterious bottom-up impacts on estuarine fish. This bloom suppression may also occur in other estuaries that receive large amounts of anthropogenic NH4. In 2010 two rare diatom blooms were observed in spring in Suisun Bay (followed by increased abundances of copepods and pelagic fish), and like the prior bloom observed in 2000, chlorophyll accumulated after NH4 concentrations were decreased. In 2010, low NH4 concentrations were apparently due to a combination of reduced NH4 discharge from a wastewater treatment plant and increased river flow. To understand the interactions of river flow, NH4 discharge and bloom initiation, a conceptual model was constructed with three criteria; 1) NH4 loading must not exceed the capacity of the phytoplankton to assimilate the inflow of NH4, 2) the NH4 concentration must be ≤4 μmol L−1 to enable phytoplankton NO3 uptake, 3) the dilution rate of phytoplankton biomass set by river flow must not exceed the phytoplankton growth rate to avoid “washout”. These criteria were determined for Suisun Bay; with sufficient irradiance and present day discharge of 15 tons NH4–N d−1at the upstream wastewater treatment plant (WTP). The loading criterion requires phytoplankton NH4 uptake to exceed 1.58 mmol m−2 d−1; the concentration criterion requires river flow >800 m3 s−1 at the WTP for sufficient NH4 dilution and the washout criterion requires river flow at Suisun Bay <1100 m3 s−1. The model and criteria are used to suggest how a reduction in anthropogenic NH4, either by reduced discharge or increased dilution (river flow), could be used as a management tool to restore pre-existing productivity in the SFE and similarly impacted estuaries. [Copyright &y& Elsevier]

Abstract: The equatorial Pacific plays a central role in the global carbon cycle as the largest oceanic source of CO2 to the atmosphere. The region has been described as a HNLC (High Nutrient, Low Chlorophyll) region with low chlorophyll being explained by either Fe limitation, Si(OH)4 limitation, or a combination of the two. The autotrophic community in the equatorial upwelling zone (EUZ) is dominated numerically by small cell-sized picoplankton but also contains larger cells, including diatoms that require Si(OH)4 and have been hypothesized to drive NO3 uptake. The 1-D CoSiNE (Carbon, Silicon, Nitrogen Ecosystem) model was developed for the EUZ to evaluate the potential drivers of the HNLC condition. The model assumes two phytoplankton groups (small non Si(OH)4 users and larger Si(OH)4 requiring cells). Many of its predicted variables have not been measured in the field -- specifically, uptake of NO3, NH4 and Si(OH)4 by the larger cells versus the smaller picoplankton and biomass values (as particulate nitrogen, PN) of the two size classes. Two cruises to the EUZ were undertaken in December 2004 (EB04) and September 2005 (EB05) to collect these data, in part to test the conceptual basis and functioning of the CoSiNE model. These EUZ cruises were unique in having size-fractionated uptake rates and phytoplankton community composition at the same depths throughout the euphotic zone and at many stations representing a range of environmental conditions and community composition. Here, we report results pertaining to NO3 and NH4 uptake, PN, and f-ratio by different size fractions of the phytoplankton community collected during zonal (0°Eq and 0.5°N) and meridional surveys at 110°W and 140°W. Larger sized (>5μm) phytoplankton contributed more to NO3 uptake (up to 85%) than cells <5μm and so had higher f-ratios. Several autotrophic functional groups were likely responsible for NO3 uptake in the >5-μm size class, including both diatoms and dinoflagellates. The importance of diatoms in NO3 uptake appears to be variable, with biological “hotspots” around regions of locally enhanced upwelling associated with tropical instability waves (TIWs). In some cases, TIW activity favored diatoms, and Si(OH)4 and NO3 uptake were elevated strongly during TIW activity. Largely as a result of increased diatom productivity, integrated NO3 uptake rates during the passage of these waves reached 6.74mmolm−2 d−1, exceeding the mean values previously reported for the equator by >2-fold. NH4 concentrations and uptake were also dynamic, with lower values south of the equator and higher values to the north. The major contributors to NH4 uptake were the smaller (<5μm) phytoplankton that resulted in low total community f-ratios of 0.26 over both years. A >2-fold increase in NH4 uptake rates in 2005 was consistent with the observed roughly 2-fold increase in Prochlorococcus biomass. Our results highlight a potential area for improvement to the CoSiNE model, to include the role of autotrophic dinoflagellates. [Copyright &y& Elsevier]