No abstract

Six species of phytoplankton recently isolated from upper San Francisco Bay were tested for their sensitivity to growth inhibition by ammonium (NH 4 +), and for differences in growth rates according to inorganic nitrogen (N) growth source. The quantum yield of photosystem II (Fv/Fm) was a sensitive indicator of NH 4 + toxicity, manifested by a suppression of Fv/Fm in a dose‐dependent manner. Two chlorophytes were the least sensitive to NH 4 + inhibition, at concentrations of >3,000 μmoles NH 4 + · L−1, followed by two estuarine diatoms that were sensitive at concentrations >1,000 μmoles NH 4 + · L−1, followed lastly by two freshwater diatoms that were sensitive at concentrations between 200 and 500 μmoles NH 4 + · L−1. At non‐inhibiting concentrations of NH 4 +, the freshwater diatom species grew fastest, followed by the estuarine diatoms, while the chlorophytes grew slowest. Variations in growth rates with N source did not follow taxonomic divisions. Of the two chlorophytes, one grew significantly faster on nitrate (NO 3 −), whereas the other grew significantly faster on NH 4 +. All four diatoms tested grew faster on NH 4 + compared with NO 3 −. We showed that in cases where growth rates were faster on NH 4 + than they were on NO 3 −, the difference was not larger for chlorophytes compared with diatoms. This holds true for comparisons across a number of culture investigations suggesting that diatoms as a group will not be at a competitive disadvantage under natural conditions when NH 4 + dominates the total N pool and they will also not have a growth advantage when NO 3 − is dominant, as long as N concentrations are sufficient.

Impacts on growth rates from exposure to ammonium (NH 4 +) and nitrate (NO 3 −), at non-limiting concentrations, in combination with irradiances varying from 25 to 600 µmol photons m −2 s −1 were investigated in the pelagic diatom Thalassiosira weisflogii and the benthic diatom Entomoneis paludosa recently isolated from Suisun Bay in northern San Francisco Bay. Growth rates were higher in T. weisflogii (0.76 ± 0.3 d −1) compared with E. paludosa (0.58 ± 0.2 d −1) across all irradiances and nitrogen (N) treatments. Differences in growth rates with N source were regulated by irradiance in both diatoms and were greatest at the intermediate irradiance due to saturation of rates at 85 µmol photons m −2 s −1 when growing on NH 4 + and at 200 µmol photons m −2 s −1 when growing on NO 3 −. Notable physiological differences between these 2 diatoms included a larger range in the quantum yield of photosystem II (F v /F m) and in chlorophyll a per cell as a function of irradiance in T. weissflogii compared with E. paludosa. In addition, a negative interaction of high NH 4 + concentration (1000 µmol l −1) and irradiance (≥200 µmol photons m −2 s −1) was observed on growth rates in E. paludosa that was not evident in T. weissflogii. Differences in physiological parameters of these diatoms are discussed in relation to their distributions and frequency of occurrence in Suisun Bay.

Since the 1980s, the San Francisco Bay Delta ecosystem has experienced large declines in primary production. Hypothesized reasons for this decline include (1) suppression of nitrate (NO3−) uptake, and thus phytoplankton growth, due to high concentrations of ammonium (NH4+), and (2) wastewater NH4+-induced changes in phytoplankton community composition away from large-celled diatoms. These twin hypotheses implicate NH4+ loading from the Sacramento Regional Wastewater Treatment Plant effluent outfall in explaining declines in primary production in the region. They have been controversial within the water resources management community and have stimulated a lengthy public scientific and regulatory debate. Here, in an effort to resolve this debate, we present results from a 48-h incubation experiment with surface water from both upstream and downstream of the Sacramento Regional Wastewater Treatment Plant effluent outfall, a major source of NH4+ loading to the ecosystem. We amended this water with either NH4+, NO3−, or full wastewater effluent. All assays were incubated under high light (52% of incident irradiance) or low light (6% of incident irradiance). NO3− uptake rates were suppressed to near zero in all treatments with either added NH4+, added wastewater effluent, or high in situ NH4+ concentrations. Yet, phytoplankton uniformly grew well on all dissolved inorganic nitrogen sources, including effluent and NH4+. Diatom species were the most abundant taxa at all stations, and diatom cell abundances increased at greater rates than all other taxa over the course of the experiment. Among all treatments, the light treatment had the greatest effects on chlorophyll a accumulation and phytoplankton growth rates. Our results suggest that high NH4+ loading is not a driver of the lower productivity in the San Francisco Bay Delta. Although phytoplankton preferred NH4+ to NO3− when both were available in our experiment, the form of dissolved inorganic nitrogen had no effect on growth rates or species composition.

Salps are pelagic tunicates that are able to form large blooms under favorable conditions by alternating between sexual and asexual reproduction. While their role in the regional carbon cycle is receiving attention, our knowledge of their physiology is still limited. This knowledge gap is mainly due to their fragile gelatinous nature, which makes it difficult to capture intact specimens and maintain them in the laboratory. We present here a modified kreisel tank system, that was tested onboard using the Southern Ocean salp Salpa thompsoni and station- based using the Mediterranean species Salpa fusiformis. Successful maintenance over days to weeks allowed us to obtain comparable relative growth and developmental rates as in situ, and provided insight into their potential life cycle strategies. By providing a starting point for successful maintenance, we hope to stimulate future experimental research on this understudied taxonomic group.