Phytoplankton dynamics in the Salton Sea, California, 1997–1999

Abstract The dynamics of phytoplankton populations in the Salton Sea were studied over the 3-year period 1997–1999. Dino-flagellates were important components, often contributing over 80% of the total biovolume of cells larger than 5 μm. Gyrodinium uncatenum along with a similar, but rarer species Gyrodinium instriatum, was the most dominant taxon, present year-round, and became especially abundant in spring and summer. Diversity of dinoflagellates was high with three or four species often co-occurring with similar densities. The largest dinoflagellate, Gonyaulax grindleyi (= Protoceratium reticulatum), increased greatly in abundance during 1997–1999. Diatoms co-dominated year round with the colonial araphid Thalassionema sp. also becoming especially abundant in 1999. During the winter mixing period, diatom populations often were dominated by pennate species usually considered benthic, such as Pleurosigma ambrosianum, Ceratoneis closterium and Tryblionella punctata. Cryptomonads, represented by several species, were usually numerous and, although relatively small, constituted as much as 20–30% of total phytoplankton biovolume. A raphidophyte, Chattonella marina, reported to be ichthyotoxic in other locales, was abundant in summer, comprising about a third of total phytoplankton biovolume then and reaching mid-lake densities of nearly 1,500 cells ml−1. Two colonial non-motile green algae were usually present; one of these, Crucigenia rectangularis, increased greatly in density in 1999. A euglenoid, Eutreptia lanowii, was highest in density in summer when it contributed about 5–10% of total biovolume at times after other species were reduced by sulfide events. Filamentous planktonic cyanobacteria were very rare. The increase in large species of phytoplankton over the three-year period coincided with a dramatic decrease in abundance of a planktivorous fish, the Mozambique mouthbrooder (Oreochromis mossambicus x O. urolepis honorum), which likely was responsible for these changes in phytoplankton composition. Phytoplankton biovolume densities and chlorophyll a concentrations each year were highest, 6–16 mm3 l−1 and 30–40 μg 1−1 respectively, in the spring. In summer and early fall, occasional windstorms mixed the water column causing sulfide-laden bottom waters to upwell and strip surface waters of oxygen. Sharp drops in abundance of most phytoplankton species occurred during these events presumably due to poisoning by hydrogen sulfide. Satellite images confirm the presence of large patches of high albedo surface waters at these times, lasting days to over a week, produced by backscattering from abundant crystals of gypsum produced following oxidation of hydrogen sulfide to sulfate.