ABSTRACT. Stable carbon and nltrogen isotope data, complen~ented wlth other geochemical parameters, were used to Identify the sources of organic mattrr that support the food web of an anchialine cave ecosystem In the northeastern Yucatan Peninsula, Mexlco. A n c h~a l~n e caves, coinmon along tropical kal-st~c and volcan~c coastlines, are completely or partially Inundated by highly stratified layers of fresh and marine waters. Stable isotope data from the cave fauna, the particulate organlc matter (POM) from the cenote pool and from the cave, the forest soil and the cave sedlmcnts ~ndicated that at least 3 soul-ces of nutritive organics could support the anchialine food web. Thcsr sources were. (1) soil from the overlying forest; (2) freshwater algae from adjoining open water pools; and (3) chernoautotrophic nitrifying bactei-la living in the cave. Production of nitrate a n d a tlccrcasc in O2 along the halocline provided geochemlcal evidence of nitrification. Stable nitrogen isotope datd defined 2 to 2.5 trophic levels In the food web. Furthermore, it was found that troglobitic (cave-limited) species res~ding In the water column a r e cilpable of preferentially feeding on specific organic reservoirs. This study presents the first extensive description of the ecological and biogeochemical relationships of the anchialine cave ecosystem.

ABSTRACT:The carbon isotope ratio of particulate carbon in an estuary can vary by at least 10 %O throughout the year. In this study. 2 periods of high primary productivity are compared from data collected in both spring and summer of 1987 and 1988 from the Delaware Estuary (USA). In spring. particulate carbon isotopic compositions (6I3C) were the most positive (up to -17 %0), whereas in summer the values were the most negative (to -32 %o). Equations calculating the CO, concentrations within algal cells were used to show that growth of phytoplankton, especially in spring, may be limited by the availability of dissolved carbon dioxide, C02(d)-An alternative hypothesis for the enrichment of I3c in diatoms from spring blooms includes the possibility that a mechanism for active bicarbonate accumulation is induced during high primary productivity. Similarly, a model is developed for phytoplankton growth during summer with C02(d), rather than bicarbonate, being the species of dissolved inorganic carbon (DIC) transported across the membrane. The influence of respired CO2 on the isotopic composition of total DIC in summer is also calculated to explain the differences in isotopic compositions of particulate carbon. The demand for during periods of high primary productivity limits its availability to phytoplankton. This demand could easily be met by the induction of an active transport system for concentrating DIC, which could explain some of the variability in 6'" of partlculate carbon In estuaries and oceans.

Variation in the distribution of stable nitrogen isotopes among different size classes of particulate organic nitrogen (PON) in aquatic environments may be partly explained by isotope effects associated with regeneration of NH,* by the microbial food web. Protists fed the marine bacterium Vibno natl-ieg~ns were grown in batch and continuous culture to define the isotope discrimination between NH,', microbial biornass, and dissolved organic nitrogen (DON) for culture systems closed or open to exogenous nutrient inputs. In batch cultures of a flagellate Pseudobodo sp. and a scut~cocil~ate Uronema sp., the nitrogen isotope discriminat~on between b~omass and NII.,' was 3 to 5 "6, during exponential growth of protists corresponding to the highest rates of b~omass-specific NH,' release The 6"N of D O N , calculated by mass balance, was very depleted in 15N relative to NIi,' and biomass dunng exponential and early stationary growth when D O N and dissol\ied free amino acid (DFAA) concentrations increased. In contrast to batch culture results, for continuous culture of the flagellate the Isotope discrimination between biomass and NH,' and the change in the calculated 6I5N of DON were small. 1 to 2 X,. Based on rates of thymidine incorporation and turnover of DFAA, protist excretion and bacterial uptake of DON were less coupled for exponentially growing protists fed starved bacteria in batch cultures than for the flagellate fed active bacteria growing in a 2-stage continuous culture. Coupling between release and utilization of DON, as well as isotope effects associated with protist metabolism, are proposed to constrain the nitrogen isotope dynamics of NH,', suspended PON, and DON within marine ecosystems, especially those environments dominated by regenerated primary production.