Salinity and fish effects on Salton Sea microecosystems: water chemistry and nutrient cycling

A 15 month long experiment was undertaken to document responses of the Salt on Sea biota to experimentally manipulated salinity levels (30, 39, 48, 57, and 65 g l(-1)) in 312-liter fiberglass tanks maintained outdoors. At two salinities (39 and 57 g l(-1)) microcosms were set up each having one small tilapia (Oreochromis mossambicus) in order to assess its influence on the system. To 28 tanks filled with Salton Sea water diluted to 30 g l(-1), dif ferent salts (NaCl, Na2SO4, MgSO4. 7H(2)O, KCl) were added in constant prop ortions to produce the desired salinity levels. Salton Sea shoreline sedime nt was added to the bottom of each tank, and inocula of algae and invertebr ates were added on several occasions. Invertebrate populations, phytoplankt on, periphyton, and water chemistry were monitored at regular intervals. Th is article present the results concerning water chemistry and nutrient cycl ing. There was no apparent increase in salinity over time, though similar to 119 0 l of tapwater with a salinity of similar to 0.65 g l(-1) added to each ta nk during the experiment. Ionic composition varied both among treatments an d over time to some degree. Ca-2 concentrations were the same at all salini ties, while K-1 concentrations were > 3 times greater at the highest salini ty than at the lowest. pH showed little consistent variation among saliniti es until the last few months when it was higher by similar to 0.4 units at the two higher salinities than at the lower ones; it was unaffected by fish . Absolute oxygen concentrations were negatively correlated with salinity, and occasionally depressed by the presence of fish. PO43-, dissolved organi c phosphorus, and particulate phosphorus concentrations were often reduced by 30-80% at 65 g l(-1) relative to lower salinities and by the presence of fish. Early in the experiment NO32- concentrations were > 2 times higher a t 57 and 65 g l(-1) than at lower salinities, but otherwise effects of sali nity on dissolved forms of nitrogen were not marked; particulate nitrogen w as much lower at 65 g 1-1 than at other salinities and also was reduced by up to 90% by the presence of fish. Silica concentrations increased over tim e at all salinities, but, relative to those at lower salinities, were reduc ed by 60-90% at 65 g l(-1) by abundant periphytic diatoms. The TN:TP ratio (molar basis) was 24-30 initially and 35-110 at the end of the experiment; it was positively correlated with salinity and the presence of fish. Mechan isms accounting for the above patterns involve principally the biological a ctivities of phytoplankton and periphyton, as modified by grazing by Artemi a franciscana and Gammarus mucronatus, and the feeding and metabolic activi ties of the tilapia. The large reduction in water column TN and TP levels b rought about by the fast-growing, phyto- and zooplanktivorous tilapia sugge st that amelioration of the Salton Sea's hypereutrophic state might be assi sted by a large scale, sustained yield fish harvesting operation.