Nutrient dynamics in ship harbour,Nova Scotia

Abstract

An intensive study of the water column distributions of the dissolved nutrients silicate, phosphate, nitrate, ammonia and of dissolved oxygen in Ship Harbour, Nova Scotia in 1991 and 1992, has revealed details of, and some of the mechanisms responsible for, nutrient distributions in temperate inlets. Ship Harbour is ～10km by ～1 km wide and has a deeper (～25 m) inner basin separated from coastal waters by a shallow sill (～7 m). Nutrient distributions here result from a combination of processes within the inlet and the typical seasonal cycle expected for temperate shelf waters. Different sources and processes control each of the nutrients. In Ship Harbour, offshore nutrient supply is important for nitrate and phosphate, but is not important for silicate and ammonia. Biological uptake during the spring is important for nitrate, silicate and phosphate. Remineralization of organic matter in the deep water or the sediments of the inner basin causes concentrations of silicate, phosphate and ammonia to be unusually high, and dissolved oxygen to be unusually low, in the deep water of the inner basin in the late summer and fall. A time‐dependent model of this regeneration shows that the partial isolation of the inner basin, whose turnover time is no longer than a few months, is the determining factor. Neither the supply of organic matter to inner basin sediments, nor the ensuing regeneration rates, is unusual for inlets in the region. This finding is consistent with other recent work showing the dominance of sills in determining eutroph‐ication status of inlets in the region. In Ship Harbour, high silicate levels are found in surface waters near the head of the inlet throughout the year, despite the fact that the concentrations of reactive silicate in the rivers entering the inlet are very low. Even so, the discharges of the rivers are still the most likely external source for the silicate in the surface waters of the inlet, which must be delivered in a non‐reactive form and are probably cycled through sediments near the head of the inlet. Finally, calculations based on the biomass of cultured mussels in Ship Harbour and their clearance rate show that mussels can filter ～5–20% of the volume of the inner basin per day and, in recent years, may be starting to have a detectable impact on ambient levels of ammonia and phosphate in the inner basin.