Tidal and seasonal nutrient dynamics and budget of the Chupa Estuary, White Sea (Russia)

Abstract

This study is the third in a series of papers on the Chupa Estuary, White Sea, Russia, and investigates tidal and seasonal fate and fluxes of materials from a small fjord-like Arctic estuary to the coastal zone. Respective companion papers have detailed the hydrography (Estuar. Coast. Shelf Sci. 48 (1999) 1) and metals in sediments and mussels (Estuar. Coast. Shelf Sci. 48 (1999) 13). Two seasonal cruises were undertaken in summer (July 1994) and autumn (September, 1995). Nutrients (nitrate, nitrite, phosphate and silicate) and dissolved oxygen were recorded along the estuarine longitudinal axis and at diurnal anchor stations at key points in the estuary. The fluvial nutrient input was negligible throughout. Nutrient concentrations showed considerable inter-tidal and spring–neap variability. Evidence from isopycnals showed that this variability is related to water exchange processes in the deep basins where renewal takes place within the space of a tidal cycle. Deep-water renewal and cascading is more pronounced on neap tides due to reduced turbulent mixing. The downwelling of seawater ensures that the deep basin waters are well ventilated, and provides a means of coupling between bottom and surface water nutrient pools whereby remineralized nutrients are dispersed and recirculated. Nutrient fluxes to the coastal zone were quantified in autumn as a net import of dissolved inorganic silicate (DSi) and dissolved inorganic phosphate (DIP) at 2.10 and 0.51 mol s −1, respectively, and a dissolved inorganic nitrate + nitrite (DIN) export of 1.72 mol s −1. In July DIN, DIP and DSi were exported at 3.57, 0.41 and 7.04 mol s −1, respectively. An assessment of hydrodynamical parameters shows that the circulation is complex throughout the estuary, and so the resolved fluxes are semi-quantitative. Fjord-like circulation results in a large export of inorganic nutrients in the lower nutrient-rich layers compared to the incoming intermediate layers, which drives the system towards N-limitation in both summer and autumn, thus lending support to the concept of N-limiting tendencies observed in other sub-Arctic areas.