The increased frequency and intensity of drought with climate change may cause an increase in the magnitude and toxicity of freshwater cyanobacteria harmful algal blooms (CHABs), including Microcystis blooms, in San Francisco Estuary, California. As the fourth driest year on record in San Francisco Estuary, the 2014 drought provided an opportunity to directly test the impact of severe drought on cyanobacteria blooms in SFE. A field sampling program was conducted between July and December 2014 to sample a suite of physical, chemical, and biological variables at 10 stations in the freshwater and brackish reaches of the estuary. The 2014 Microcystis bloom had the highest biomass and toxin concentration, earliest initiation, and the longest duration, since the blooms began in 1999. Median chlorophyll a concentration increased by 9 and 12 times over previous dry and wet years, respectively. Total microcystin concentration also exceeded that in previous dry and wet years by a factor of 11 and 65, respectively. Cell abundance determined by quantitative PCR indicated the bloom contained multiple potentially toxic cyanobacteria species, toxic Microcystis and relatively high total cyanobacteria abundance. The bloom was associated with extreme nutrient concentrations, including a 20-year high in soluble reactive phosphorus concentration and low to below detection levels of ammonium. Stable isotope analysis suggested the bloom varied with both inorganic and organic nutrient concentration, and used ammonium as the primary nitrogen source. Water temperature was a primary controlling factor for the bloom and was positively correlated with the increase in both total and toxic Microcystis abundance. In addition, the early initiation and persistence of warm water temperature coincided with the increased intensity and duration of the Microcystis bloom from the usual 3 to 4 months to 8 months. Long residence time was also a primary factor controlling the magnitude and persistence of the bloom, and was created by a 66% to 85% reduction in both the water inflow and diversion of water for agriculture during the summer. We concluded that severe drought conditions can lead to a significant increase in the abundance of Microcystis and other cyanobacteria, as well as their associated toxins.

This paper reviews and highlights recent research findings on food web processes since an earlier review by . We conduct this review within a conceptual framework of the DeltaSuisun food web, which includes both temporal and spatial components. The temporal component of our framework is based on knowledge that the landscape has changed markedly from historical conditions. The spatial component of our framework acknowledges that the food web is not spatially static; it varies regionally and across habitat types within regions. The review highlights the idea of a changing baseline with respect to food web function. New research also indicates that interactions between habitat-specific food webs vary across the current landscape. For example, based on early work in the south Delta, the food web associated with submerged aquatic vegetation was thought to provide little support to species of concern; however, data from other regions of the estuary suggest that this conceptual model may not apply across the entire region. Habitat restoration has been proposed as a method of re-establishing historic food web processes to support species of concern. Benefits are likely for species that directly access such restored habitats, but are less clear for pelagic species. Several topics require attention to further improve the knowledge of food webs needed to support effective management, including: (1) synthesis of factors responsible for low pelagic biomass; (2) monitoring and research on effects of harmful algal blooms; (3) broadening the scope of long-term monitoring; (4) determining benefits of tidal wetland restoration to species of concern, including evaluations of interactions of habitat-specific food webs; and (5) interdisciplinary analysis and synthesis. The only certainty is that food webs will continue to change in response to the changes in the physical environment and new species invasions.

The Cache Slough complex is an area of tidal sloughs in the Sacramento-San Joaquin River Delta of California (USA), and is surrounding by irrigated agricultural lands. Among the species of concern in the area is the delta smelt (Hypomesus transpacificus), a federally listed threatened species. Releases of the organophosphate insecticide chlorpyrifos and pyrethroid insecticides were examined to determine whether they represented a threat to the copepods on which delta smelt feed (Eurytemora affinis and Pseudodiaptomus forbesi) and to aquatic life in general, represented by the standard testing organism, Hyalella azteca. There was a single incident of toxicity to H. azteca as a result of discharge of agricultural irrigation water containing chlorpyrifos. Pyrethroids were not found in samples collected during the dry season. Following rain events, however, the waters of western Cache Slough repeatedly became toxic to H. azteca because of the pyrethroids bifenthrin and cyhalothrin. The 96-h median lethal concentrations (LC50s) for E. affinis and P. forbesi for the pyrethroids bifenthrin and cyhalothrin were 16.7 ng/L to 19.4 ng/L when tested at 20 °C. However, their LC50s may be 5 mg/L to 10 ng/L at in situ temperatures of the Cache Slough, comparable to the peak bifenthrin concentration observed. The dominant pyrethroid source appeared to be urban runoff entering a creek 21 km upstream of Cache Slough. Pyrethroids of urban origin were supplemented by agricultural inputs of pyrethroids and chlorpyrifos as the creek flowed toward Cache Slough.

1. Increasing blooms of Microcystis aeruginosa have unknown impacts on the copepods Eurytemora affinis and Pseudodiaptomus forbesi, which are the dominant zooplankters and key prey species for endangered larval fish in the upper San Francisco Estuary. 2. Laboratory feeding experiments were designed to measure the effect of Microcystis on copepod survival and to distinguish the effects of toxicity and nutrition. In a series of survival tests, copepods were fed a mixed diet of algae plus one of two strains of Microcystis, either producing (MC+) or lacking microcystin (MC)). 3. Microcystis significantly reduced survival even when it was a small proportion of the diet, indicating that toxicity was the major cause of mortality. Contrary to expectation, however, the MC+ strain did not result in higher mortality, suggesting that non-MC metabolites of Microcystis can be toxic to copepods. 4. Across treatments, survival of P. forbesi was greater than that of E. affinis, although the two copepods responded differently to both the ratio and the strain of Microcystis in their food. Survival of P. forbesi was greater on the MC+ strain and was inversely proportional to the ratio of dietary Microcystis (MC+ or MC)). In contrast, survival of E. affinis declined similarly across treatments and was not related to the proportion or strain of dietary Microcystis. Results indicate that the copepod P. forbesi can coexist with Microcystis while the other copepod E. affinis cannot. 5. Regardless of species, dietary Microcystis caused significant mortality to copepods, and it may cause adverse impacts to the potentially food-limited zooplankton community of the San Francisco Estuary. These impacts may not be related to the cellular MC concentration because Microcystis contains other metabolites that negatively affect copepods.