No abstract

The parameters of the structure and functioning of the ecosystem in the lagoon of Venice (Italy) were estimated in September-October 1993 at 36 stations. Phytoplankton was dominated by its nano-and pico-fractions, which comprised 60 to 99":# of the total phytoplankton biomass. Phytoplankton wet biomass varied between 100 and 600 mg m-.' and its primary production between 30 and 90 mg C m ' d.' The nume~ical abundance of bactel-ioplankton (3 to 6 X 10" cells ml-'l and its wet biomass (0.3 to 1 g m-" was 2 to 3 times greater than in adjacent waters of the Adriatic. Bacterial production was 20 to 40 mg C m-'d.'. hdicrozooplankton was dominated by cillates with blo~nasses of 30 to 700 mg m-', while the density of rotifers and nauplii was low. The composition of mesozooplankton resembled that of the Adriatic complex only near the port entrances connecting the lagoon to the sea, while in polluted areas i t was drastically depleted. Its average wet biomass was 80 to 300 mg m-', attaming 700 to 1100 mg m-' near the port entrances. The total plankton biomass was 0.8 to 2 g m-', and was dominated by microplankton. The stock of labile organic matter in the lagoon was 2 to 3 times greater than In the waters of the adjacent Adriatic, wlth a turnover time w h~c h varled between 10 and 20 d. The stock of labile sulfldes in the upper layer of the bottom sediments in the central basin of the lagoon was 1.5 to 2 g S d m ' of wet silt. Data were generalized using the approach of energy balances and energy flows. The latter were used to evaluate the present state of the ecosystem of the lagoon of Venice, its changes due to anthropogenlc Impact, and its capability for self-purlf~cation.

Isolated posterior gills of shore crabs, Carcinus maenas, previously acclimated for at least 1 month to brackish water of 10 %o S, were connected with an artificial hemolymph circulation by means of thin polyethylene tubings. Gills were symmetrically perfused and bathed with 50 % sea water. Transepithelial potential differences (PDs) and fluxes of sodium between medium and blood were measured under control conditions and following reductions of PDs by means of 5 mM internal (blood side) ouabain, 0.5 mM internal and external (bathing medium) NaCN or by exhaustion of energy reserves along with a prolonged perfusion period of more than 9 h. In these experiments 22Na was used as tracer. Each of the three modes of reducing transepithelial potential differences resulted in a decrease in sodium influxes from 500-1000 [tmoles g-1 h-1 to 250-400 F, moles g ~ h -I. The findings suggest that sodium influx, which normally greatly exceeds efflux, was diminished by its active component. The remaining non-inhibitable influx equals efflux values. Our findings thus indicate that efflux is completely passive, while influx has -beside a passive component of efflux magnitudes -an additional active portion which is much larger than the passive component. Since ouabain is a specific inhibitor of the Na-K-ATPase, our results confirm previous findings (Siebers et al., 1985) that the basolaterally located Na-K-ATPase generates the transepithelial potential difference in the gills, which is inside negative by about 6-12 mV. Inhibition of the active portion of sodium influx by internal ouabain along with reduced PDs suggests that transepithelial PDs generated by the branchial sodium pump are the driving force for active sodium uptake in hyperregulating brackish water crabs.

No abstract

Fine structural studies were conducted on the gills of the shore crab Carcinus mediterraneus using scanning electron microscopic techniques. The results obtained show the structural organization of crab gills from whole gills including spiny elements over the 150 lameUae to lamellar components such as cuticles, median shaft, marginal canal, afferent and efferent lameltar vessels and hemolymph cells. Enormous surface enlargement is accomplished by a variety of structural elements which allow rapid circulation of hemolymph. In the form of a relatively small organ, the gills fulfill all the necessary exchanges of specific molecules between the crab and its environment. Aggregations of ca 1-~tm particles covering the outer cuticular surfaces are considered to be bacterial colonies of unknown properties and functions.