EXPERIMENTAL MESOCOSM STUDIES OF SALINITY EFFECTS ON THE BENTHIC ALGAL COMMUNITY OF A SALINE LAKE

As closed-basin systems, saline lakes are prone to fluctuate in level and salinity with climate change and hydrologic alterations. Loss of m any Great Basin Cakes has resulted from the diversion of tributary str eams for agricultural or municipal uses. At Mono Lake, an alkaline sal t lake in eastern California, salinities have risen from 50 to 100 g.L -1 in just 50 years. Experimental mesocosms were established to simula te some of the potential ecological effects that could have accompanie d this change. The influence of salinity on diatom diversity,taxonomic structure, and primary Production was tested using mesocosms deployed at Mono Lake. Mesocosm tanks were 500 L in volume, 1 m square, and 0. 5 m deep, with open tops covered by I mm mesh net. I;ive treatments (5 0, 75, 100, 125, and 190 p.L-1) with four replicates per treatment wer e used over a 2-month period. The diatom-dominated benthic algae were reduced both in standing crop (from 6 to <0.1 g.m(-2)) and diversity ( from 30 to 12 taxa) with increased salinity, with most loss occurring in salinities greater than or equal to 75 g.L-1. Photosynthetic oxygen production also was significantly lower at salinities greater than or equal to 75 g.L-1. Diatom indicator taxa for these shifts included De nticula sp., Nitzschia frustulum, N. monoensis, N. communis, and Steph anodiscus oregonicus increasing in relative abundance in higher salini ty treatments, accompanied by decreases in Achnanthes minutissima, Cym bella minuta, N. dissipata, and Rhoicosphenia abbreviata. Exhibiting d ominance at moderate salinity levels (75 to 125 g.L-1) were Nitzschia frustulum, N. communis, N. palea, and Navicula crucialis. These latter species may be limited by both physiological stress at high salinity and grazing and competition at low salinity. The filamentous chlorophy te, Ctenocladus circinnatus, and cyanobacteria (Oscillatoria spp.) occ urred only in salinity treatments from 50 to 100 g.L-1. Diversion of t ributary stream flow and resulting salinity increases in this lake thr eaten sustained benthic primary production and algal species diversity relative to conditions prior to stream diversion. The 1994 decision o f the California State Water Resources Control Board to return stream flows to Mono Lake will raise the lake level and reduce salinity to ar ound 75 g.L-1 and is expected to increase the diversity and productivi ty of the benthic algae of this ecosystem.