Since 1933 the salinity of Pyramid Lake, Nevada, U.S.A., has increased 32% to nearly 5.5%,. We tested the hypothesis that further increases of 1.5 to 2 times (l.5X to 2X) its present salinity would significantly reduce species richness and alter population structures of the existing crustacean zooplankton community. Three strategies were applied: in addition to monitoring zooplankton in semicontrolled indoor microcosms at IX, 1.5X and 2X and conducting range-finding, acute, and chronic salinity bioassays, the present zooplankton community of Walker Lake (2X) was compared with that existing in Pyramid Lake (1X).Ceriodaphnia quadrangula and Diaphanosoma leuchtenbergianum, both collected from Pyramid Lake, were lacking in Walker Lake. Populations of Cyclops vernalis were significantly lower and those of Diaptomus sicilis and Moina hutchinsoni were significantly higher in Walker Lake than in Pyramid Lake.Densities of Ceriodaphnia and Cyclops were low in microcosms at salinities >IX. Diaphanosoma could not be maintained in microcosms, regardless of salinity. Numbers of Diaptomus and Moina in microcosms were proportional to salinity level.Short-term LC50 salinities (%,) were as follows: Diaphanosoma, 6.5; Ceriodaphnia, 7.1; Diaptomus, 13.3; Cyclops, 14.8; and Moina, 17.8. Multiple-generation, chronic bioassays were run only on Cyclops and Diaptomus. Three generations of Cyclops were produced at salinities of 4.0 to 8.5%,, but not at 9.8%, or higher. Diaptomus was unable to complete three generations at salinities >9.6%,.We speculate that high salinity in Walker Lake may indirectly benefit Diaptomus by negatively affecting predatory Cyclops, and benefit Moina by causing extinction of competing salinity-intolerant Diaphanosoma and Ceriodaphnia. Except for the response of Diaptomus, results from bioassays were in general agreement with results from microcosms and with field data. Untested predator-prey interactions could be responsible for the apparent discrepancy.

Surveys were conducted in two shallow bays in the Orkney Islands, UK; Orphir Bay, an unexploited (control) site, and Bay of Ireland, a fished site, to investigate the effects of suction dredging on the resident razor clam, Ensis arcuatus, populations. A lower density and significantly smaller mean length of razor clams were present at the dredged site compared with the control site.The age of individual razor clams was estimated using internal shell microgrowth patterns, visible in acetate peels of polished and etched shell cross-sections. Ensis arcuatus are relatively slow growing animals with the two study populations characterized by old individuals and an obvious lack of juveniles, indicating populations with little resilience to disturbance. An analysis of the shell sections of razor clams from the Bay of Ireland revealed the presence of shell margin breaks, consisting of deep clefts in which sand grains were embedded in the shell matrix, whilst those from Orphir Bay had fewer disturbances to shell growth. It is suggested the disturbances to shell growth are the result of repeated suction dredging operations in the Bay of Ireland.In situ reburrowing experiments were conducted to determine the survival rate of E. arcuatus (<160 mm shell length), returned to the sea after capture and to estimate the indirect effect of dredging on the razor clam population. These individuals displayed a slow initiation of ''escape-digging'' which rendered them vulnerable to attack from predatory crabs and fish, indicating that there is likely to be a low survival rate of any returned undersized clams or ones that are disturbed and escape from the suction dredge.1998 International Council for the Exploration of the Sea

No abstract

Salinity of Pyramid Lake increased from 3.7 to 5.5%o between 1933 and 1980. Concern over future reductions in overall species richness prompted experiments to assess responses of dominant lake organisms to elevated salinity. Salinity tolerances of three important benthic invertebrates, Hyalella aztecta, Chironomus utahensis, and Heterocypris sp., were tested in controlled laboratory bioassays and also in a semi-natural environment consisting of large (47 m 3 ) mesocosms.Densities of H. azteca in mesocosms were significantly lower at salinities of 8.0 and 11.0%o compared with 5.6%o controls in year one, but not in 8.5%o salinity mesocosms in year two. The 96-h LC 50 for H. azteca was high at 19.5%o. Short-term mortalities of C. utahensis were 100% at salinities of 13.3%o and greater. Fifty-seven percent fewer larvae matured from third to fourth instar at 8.9 than at 5.5%o salinity in 17 day subacute bioassays. Furthermore, larval chironomid densities and emergence of adults from mesocosms were significantly reduced at salinities of 8.0%o and higher compared with controls. Mortality of Heterocypris sp. was 50%o at a salinity of 18.6%o in laboratory bioassays and populations in mesocosms ranged between 40 and 100% lower at salinities of 8.0 and 11.0%o than in controls.Multiple generation mesocosm experiments indicated all three invertebrates were more sensitive to elevated salinity than results of short-term bioassays. Our studies suggest populations of these invertebrates may be reduced from present levels if Pyramid Lake's salinity were to double, although none are expected to be extirpated. Food habit shifts and reduced production of lake fishes are likely consequences of salinity-induced disruption in the benthic invertebrate forage base.