Distribution of marine invertebrate larvae in the southern Kara Sea (Russian Arctic)

Within the generally oligotroph Arctic marine environment river outlets are favoured by many planktonic and benthic organisms due to their high input of organic carbon. The retention of pelagic larvae within nursery grounds and/or the ability to return to their parental grounds prior to settlement is one important factor for the persistence of benthic communities in such river influenced areas. The southern Kara Sea is strongly controlled by high freshwater inputs from the Ob and Yenisei Rivers, which create a pronounced bi-layered pycnocline with a warm fresh/brackish water layer on top and a cold high saline marine layer below. The dispersal of five meroplanktonic species and settled juveniles (the brittle star Ophiocten sericeum, and the polychaetes Micronephtys minuta, Nereimyra aphroditoides, Phyllodoce groenlandica and Prionospio cirrifera) in relation to the adult distribution patterns was investigated. For all apart from P. cirrifera the highest densities of larvae were found in the upper brackish water layer. To assess size-at-settlement, the body sizes of larvae and newly settled juveniles were estimated and compared. Dispersal patterns ranged from virtually no adaption to river run-off as in the common, stenohaline O. sericeum and M. minuta (7 ind./m**3, 459 µm) to local retention as in N. aphroditoides (7 ind./m**3, 541 µm) and P. groenlandica (0.5 ind./m**3, 1121 µm) retained by horizontal eddies created by the outflow. Adults of P. cirrifera, which were exclusively restricted to the estuary of the Yenisei River, showed a well adapted reproductive behaviour to ensure a high retention potential of their progenies. The larvae (1.5 ind./m**3, 1513 µm) were only present in the lower water layers, most probably taking advantage of the prevailing near bottom counter current retaining them within their hatching areas.

Particle analysis in the Kara Sea

In September 1999 two short-term moorings with cylindrical sediment traps were deployed to collect sinking particles in bottom waters off the Ob and Yenisei river mouths. Samples were studied for their bulk composition, pigments, phytoplankton, microzooplankton, fecal material, amino acids, hexosamines, fatty acids and sterols and compared to suspended matter and surface sediments in order to collect information about the nature and cycling of particulate matter in the water column. Results of all measured components in sinking particles point to an ongoing seasonality in the pelagic system from blooming diatoms in the first phase to a more retention system in the second half of trap deployment. Due to a phytoplankton bloom observed north of the Ob estuary, flux rates were generally higher in the trap deployed off the Ob than off the Yenisei. The Ob trap collected fresh surface-derived particulate matter. Particles from the Yenisei trap were more degraded and resembled deep water suspension. This material may partly have been derived from resuspended sediments.

(Table 3) Contents of P2O5 and total REE in phosphorites from Pacific seamonts

Changes in concentration levels and speciation of heavy metals during sedimentation on example of a typical semi-closed bay, where bottom sediments have formed due to river run-off, are under consideration. It is shown that due to desorption of mobile manganese, zinc and copper entered the bay with river suspended matter, their total contents in bottom sediments decrease and percentages of lithogenic forms increase. Contents and speciation of iron in bottom sediments are determined by its participation in coagulation of river colloids in the mixing zone and by mechanical differentiation of sedimentary material.