Freshwater-saltwater Transition Zone Research Articles

Photosynthetic and bacterial production gradients in a larval fish nursery:the St. Lawrence River transition zone

Phytoplankton biomass, primary production and bacterial abundance and production were measured across the freshwater-saltwater transition zone of the St. Lawrence River [Canada) during the seasonal period of maximum concentrations of fish larvae and macrozooplankton. The estuarine front was characterized by steep gradients in b~olog~cal s well as physical properties. Maximum turbidity and high phytoplankton biomass occurred in the well-mixed, low salinity (0.2 to 4 psu) region immediately upstream of the salt wedge. Peak zooplankton and ichthyoplankton biomass occurred within and slightly downstream of this frontal region The hypothesis that lower food chain processes were controlled exclusively by allochthonous carbon and bacterial heterotrophy was not supported. Photosynthetic rates per unit chlorophyll a [chl a) remained high across the freshwater-saltwater transition and the low light penetration was offset by a shallow mean depth of mixing. Bacterial concentrations and activity remained relatively constant across the transition, while chl a declined sharply downstream of the front, consistent with grazing losses. Photosynthesis contributed 34 to 66% of the total production (bacteria + phytoplankton). Freshwater phytoplankton advected from upstream contributed another 20 to 30%. These first-order estimates underscore the combined importance of photosynthesis plus bacterial processes within the downstream food web of large river ecosystems.

Shaly-Sand Log Analysis in an Environmental Application: ABSTRACT

Abstract A procedure to determine ground-water quality from analysis of electromagnetic induction logs was developed to detect and assess the distribution of saltwater intrusion into the alluvial Great Bend Prairie aquifer. The cross correlation between the induction and natural gamma-ray logs from a freshwater well was optimized (based on empirical statistics) to reduce shaly-sand effects — the variations on the induction log due primarily to the presence of clay minerals. The correction method obviates determination of variations of aquifer porosity or cation-exchange capacity for detailed log analysis of porewater conductance. The method was applied to sets of logs collected from a network of monitoring wells in the aquifer. The corrected induction logs (units of mS/m) were calibrated to chloride concentration (mg/L) based on a highly significant linear-regression relationship (R=0.99) between the chlorinity of well-water samples and the median values of the corrected log for the screened intervals of the wells sampled. The derived freshwater-saltwater transition zone profiles were processed by curve-fitting techniques to develop objective, quantitative estimates of key characteristics such as the saturated thicknesses of the usably fresh and unusable salt water (at the 500 mg/L chloride interface) and the determination of aquifer dispersivity. The curve-fit method also allowed incomplete transition zone profiles to be completed to the base of the aquifer by extrapolation. The transition zone characteristics provide a database for regionalization of saltwater distribution and for monitoring changes of the distribution at both local and regional scales.

Structure and Dynamics of Photosynthetic Picoplankton across the Saltwater Transition Zone of the St. Lawrence River

Changes in the biomass, community structure, and activity of photosynthetic picoplankton (PP) were measured across the turbid, freshwater–saltwater transition zone (TZ) of the St. Lawrence River. The <2 μm fraction of total Chl a dropped from 20–30% in mid-May to 15% in late June. Photosynthesis by the <2.0 μm fraction in May surface waters contributed 6–38% of the total primary production. Picoplankton dropped from 8.4 × 106 (freshwater zone, FZ) to 3.7 × 106 cells∙L−1 (TZ) in mid-May and from 17 × 106 (FZ) to 5.8 × 106 cells∙L−1 (TZ) in late June. Results implied that the picoplankton were subject to increased losses (e.g., by protozoan grazing) in the TZ. Phycoerythrin-rich cyanobacteria (PEC) dominated the PP counts in mid-May, with phycocyanin-rich forms (PCC) contributing 4–13% of the cell concentrations. Six weeks later the PP community had shifted completely to PCC dominance (60–69% of total counts). In late summer, PP contributed 6–56% of the planktonic Chl a in the St. Lawrence River system. There were large variations in the PEC/PCC ratio and the proportion of eukaryotic PP along certain reaches of the river. These observations underscore the spatially and temporally dynamic character of PP communities in the river–estuary environment.

Photoadaptation to intermittent light across the St. Lawrence Estuary freshwater-saltwater transition zone

Photoadaptation to intermittent light across the St. Lawrence Estuary freshwater-saltwater transition zone