Spring bloom sedimentation in a subarctic ecosystem

Results from a 5-yr study (1985 to 1989) in Auke Bay, Alaska show that termination of the spring bloom consistently occurred at limiting nitrate concentrations. Following nutrient exhaustion, phytoplankton sinking rates increased and displayed greater temporal variability. Threshold nitrate concentrations, approximating K_s values of the species present, were found to signal initiation of increased sedimentation. For Thalassiosira aestivalis, the threshold was ～2 μmol l^-1, while for Skeletonema costatum the threshold was ～1 μmol l^-1, suggesting genus-specific differences in sinking-rate sensitivity to nitrate exhaustion. Overall, sinking rates of the three principal genera ranked (high to low) Thalassiosira spp.> S. costatum>Chaetoceros spp., while the nitrate sensitivities of the sinking rates of the genera ranked (high to low) Thalassiosira spp.> Chaetoceros spp.> S. costatum. Thalassiosira spp. showed the most consistent sinking rate increases following nutrient impoverishment. During a bloom dominated by T. aestivalis, a decrease of cell sinking rate with depth coincided with a decrease in short-term nutrient stress as measured by intracellular nitrate pools. In addition, no correlation was found between chain length or aggregate formation and sinking rate for this species. Though we measured only small-scale cell-cell adhesion, not larger-scale marine snow formation, this supports the notion that the sinking rates of Thalassiosira spp. were controlled primarily by cell physiology. For S. costatum, however, shorter chains sank faster. The sinking behavior of the species studied here figures prominently in their pelagic ecology and in the carbon flux of coastal ecosystems, both of which are driven by short-term variability.     

Mercury and organochlorine content of Dipper Cinclus cinclus eggs in south-west Ireland: trends during 1990-1999

A sample of 124 deserted/infertile dipper (Cinclus cinclus) eggs was collected in south-west Ireland during six seasons (1990-1994 and 1999) and analysed for mercury and organochlorines. Mercury was detected in three of the 6 years but no trend was observed. DDT occurred above the limit of detection in only two eggs, one each in 1993 and 1994. In contrast, two derivatives of DDT (DDE and TDE) were found much more commonly. DDE occurred in over 87% of eggs, while TDE contamination was less widespread and reached a maximum of 60% in 1993, with none detected in 1991 or 1999. There was no trend in occurrence of DDE or TDE across years. PCB contamination was present consistently over the 10-year period, with little change in the proportion of contaminated eggs collected over time. In all years, the lowest recorded occurrence of total PCBs was 69% expressed on a formulation basis as Arochlor 1254. PCB congeners 138, 153 and 180 dominated. The congener pattern was similar across years with the exception of 1990 when congener 118 dominated. A distinct trend of high values of contaminants was observed in 1990 and 1993. In 1990, PCB 118 was the dominant contaminant, while in 1993, HEOD, DDE, PCB 138 and PCB 153 were primary contributors to the observed result. Other contaminants (HCB, PCB 101, gamma-HCH, PCB 170 and PCB 180) showed little obvious patterns between years and occurred at relatively low levels. No pattern was observed in contaminants when eggs were grouped according to river or altitude. These data provide a baseline for organochlorine contamination levels in Irish freshwater ecosystems against which future trends can be assessed.     

Biological pumps at the sediment-water interface: Mechanistic evaluation of the alpheid shrimp Alpheus mackayi and its irrigation pattern

Deep-burrowing crustaceans are common on all coasts. In a series of model experiments conducted at the Hawaii Institute of Marine Biology from March to October 1977, the effect of the alpheid shrimp Alpheus mackayi Banner and Banner, on material fluxes across the sediment-water interface has been determined. Viewed as a biological pump, the shrimp's irrigation pattern was not random, but intermittent, with a mean period of 6 min activity and 16 min resting. Plume velocities between 1 and 18 cm s^–1 were measured by seawater-resistant hot-wire anernometry at the apertures of active burrows. Based on 55 h of velocity data, mean hourly pumping rates of 0.35 liters h^–1 were calculated, with an average mechanical energy of 4000 erg h^–1 dissipated by burrow-wall friction and plume mixing. In microcosm experiments with 177 cm² sediment surface area and 120 cm² burrow-system surface, the pumping of one shrimp enhanced the release of silica from the pore water by an average factor of 5 compared to concentration gradient-driven flux across the sedimentwater interface in a control core with the same dimensions but without a shrimp. Our data suggest that real-world models of interfacial fluxes without inclusion of the macro-infauna as biological pumps will be unrealistic.     

Effects of copper on the latency of lysosomal hexosaminidase in the digestive cells of Mytilus edulis

Mytilus edulis collected from Tomales Bay, California, USA, during mid-winter 1979 were exposed to increased concentrations of dissolved copper under controlled laboratory conditions. A dose-dependent reduction in the latency of lysosomal hexosaminidase activity in digestive cells was induced after a 30 d exposure to copper. The half-time of the hexosaminidase staining reaction in sections of digestive gland from control mussels was 15.5 min; for mussels exposed to 25, 50, and 75 g Cu l^-1 it was 11.8, 8.5 and 5.5 min, respectively. In addition, the dyecoupled reaction product was seen earlier in sections from individuals exposed to 50 and 75 g Cu l^-1 (after 30 s) and 25 g Cu l^-1 (1 min) than in sections from control individuals (2.5 min). Copper accumulations were demonstrated histochemically to have the same distribution as the hexosaminidase reaction product, indicating that copper is sequestered in lysosomes. Copper concentrations in digestive gland tissue, were related to the concentrations of copper in the water to which the mussels were exposed.