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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 (F v /F m ) was a sensitive indicator of NH 4 + toxicity, manifested by a suppression of F v /F m 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., (© 2017 The Authors Journal of Phycology published by Wiley Periodicals, Inc. on behalf of Phycological Society of America.)

Phytoplankton subsidies from river inputs and wetland habitats can be important food sources for pelagic organisms in the Sacramento--San Joaquin Delta (Delta). However, while the Sacramento River is a key contributor of water to the Delta, providing 80% of the mean annual inflow, the river is only a minor source of phytoplankton to the system. The reason for low phytoplankton biomass in the Sacramento River is not well understood but appears to be associated with a 65-km stretch of the lower river where chlorophyll-a (Chl-a) concentrations can decline by as much as 90%. We conducted two surveys along the lower Sacramento River, in spring and fall of 2016, to investigate the relative contributions of different factors potentially driving this Chl-a decline. Our study evaluated the change in Chl-a concentrations as a result of dilution from tributaries, light availability, nutrient concentrations, nutrient uptake, phytoplankton productivity, zooplankton grazing, and clam grazing. Chl-a concentration decreased from 14 µg L-1 to 1.8 µg L-1 in the spring and from 4.0 µg L-1 to 1.2 µg L-1 in the fall. Dilutions from the Feather River and American River contributed 39% and 11% of Chl-a declines, respectively, during the spring. Average water depths roughly doubled downstream of the American River confluence, reducing water column light availability and lowering productivity. Zooplankton and clam grazing rates were generally low. Using a mass balance analysis, the measured variables explained 76% of the observed decline in Chl-a in the spring, suggesting additional losses from unidentified factors. We found that phytoplankton biomass is regulated by multiple potential factors in the lower Sacramento River, emphasizing the need for practitioners of restoration and management programs to evaluate multiple potential factors when attempting to enhance phytoplankton production in the Delta, or other large river systems. [ABSTRACT FROM AUTHOR]

[i] Surface ö13NPON increased 3.92 ± 0.48%over the course of 20 days following additions of iron (Fe) to an eddy in close proximity to the Antarctic Polar Front in the Atlantic sector of the Southern Ocean. The change in ö13NPON was associated with an increase in the >20 μm size fraction, leading to a maximal difference of 6.23% between the >20 μm and <20 /tm size fractions. Surface δ13Cpoc increased 1.18 ± 0.3l%o over the same period. After a decrease in particulate organic matter in the surface layer, a second phytoplankton community developed that accumulated less biomass, had a slower growth rate and was characterized by an offset of 1.56% in ö13CPOC relative to the first community. During growth of the second community, surface δ13C μc further increased 0.83 ± 0.l3%. Here we speculate on ways that carboxylation, nitrogen assimilation, substrate pool enrichment and community composition may have contributed to the gradual increase in δ3CPOC associated with phytoplankton biomass accumulation, as well as the systematic offset in δs13CPOC between the two phytoplankton communities. [ABSTRACT FROM AUTHOR]

In the Ross Sea, the prymnesiophyte Phaeocystis antarctica G. Karst. dominates deeply mixed water columns, while diatoms dominate shallower mixed layers. Understanding what controls the dynamics of these two phytoplankton taxa is essential because they dominate virtually all coastal polar waters, have different nutrient utilization characteristics, and support dissimilar food webs. We cultured two strains of P. antarctica and one strain of the diatom Fragilariopsis cylindrus (Grunow) Willi Krieg under three dynamic irradiance regimes that simulated different mixed-layer depths and measured their photosynthetic characteristics, cellular pigment concentrations, and cellular carbon and nitrogen content. In both species, chl a-normalized maximum carbon uptake rate ( P ) and specific growth rate were highest in the deeply mixed treatment that had a dark period. In all irradiance treatments, both ( P ) and photosynthetic efficiency (α*) were greater for the two P. antarctica strains than for the F. cylindrus strain. In contrast, P. antarctica strains were more susceptible to photoinhibition (β*) than the F. cylindrus strain. When photosynthetic rates of each phytoplankton taxon were normalized by cellular particulate organic carbon (POC), the difference in the maximal photosynthetic rate () was generally reduced. In the dynamic irradiance treatment that simulated the shallowest mixed-layer irradiance, all three phytoplankton had similar ; however, the diatom had a 2-fold higher POC-normalized photosynthetic efficiency (α). Finally, we performed calculations using the measured POC-normalized photosynthetic parameters to show that α and can play a greater role than β in determining the competitive outcome between P. antarctica and F. cylindrus in both shallow and deep mixed-layer environments of the Ross Sea. [ABSTRACT FROM AUTHOR]

San Francisco Bay (SFB), USA, is highly enriched in nitrogen and phosphorus, but has been resistant to the classic symptoms of eutrophication associated with over-production of phytoplankton. Observations in recent years suggest that this resistance may be weakening, shown by: significant increases of chlorophyll- a ( chl-a ) and decreases of dissolved oxygen (DO), common occurrences of phytoplankton taxa that can form Harmful Algal Blooms (HAB), and algal toxins in water and mussels reaching levels of concern. As a result, managers now ask: what levels of chl-a in SFB constitute tipping points of phytoplankton biomass beyond which water quality will become degraded, requiring significant nutrient reductions to avoid impairments? We analyzed data for DO, phytoplankton species composition, chl-a, and algal toxins to derive quantitative relationships between three indicators (HAB abundance, toxin concentrations, DO) and chl-a . Quantile regressions relating HAB abundance and DO to chl-a were significant, indicating SFB is at increased risk of adverse HAB and low DO levels if chl-a continues to increase. Conditional probability analysis (CPA) showed chl-a of 13 mg m −3 as a “protective” threshold below which probabilities for exceeding alert levels for HAB abundance and toxins were reduced. This threshold was similar to chl-a of 13–16 mg m −3 that would meet a SFB-wide 80% saturation Water Quality Criterion (WQC) for DO. Higher “at risk” chl-a thresholds from 25 to 40 mg m −3 corresponded to 0.5 probability of exceeding alert levels for HAB abundance, and for DO below a WQC of 5.0 mg L −1 designated for lower South Bay (LSB) and South Bay (SB). We submit these thresholds as a basis to assess eutrophication status of SFB and to inform nutrient management actions. This approach is transferrable to other estuaries to derive chl-a thresholds protective against eutrophication. [ABSTRACT FROM AUTHOR]