Abstract
Otago Harbour is a long (23 km), narrow (mean width = 2 km), shallow (mean water depth = 4.5 m) tidal inlet covering 46 km(2) on the southeast coast of South Island, New Zealand (45 degrees 50'S, 170 degrees 35'E). Development of the City of Dunedin (pop. 125,000) and its associated port at Port Chalmers has been associated with extensive dredging, land reclamation, and shoreline construction. Here we develop a carbonate sediment budget for Otago Harbour, with limits defined at Mean High Water Spring and the harbour entrance; from the water-air interface to a few cm below the sediment-water interface. Carbonate is added to this system primarily by in-situ production (similar to 10,000 tonnes CaCO3 y(-1)) and by transport though the harbour entrance from the longshore system ( similar to 24,000 tonnes CaCO3 y(-1)). Shellfishing (similar to 2 tonnes - CaCO3 y(-1)), dredging (similar to 18,000 tonnes CaCO3 y(-1)), and early sea-floor processes such as abrasion and dissolution (similar to 2000 tonnes CaCO3 y(-1)) remove carbonate from the system. The present-day carbonate budget results in similar to 14,000 tonnes CaCO3 y(-1) sediment storage, equivalent to similar to 0.14 mm y(-1) accumulation. Two thousand years ago, the budget would have had nearly the same inputs but many fewer outputs, potentially resulting in storage twice what it is today; projected increases in human impacts suggest that carbonate storage may end within 100 years. Carbonate storage in sediments has a role in preserving environmental information and sequestering carbon, but the major value of a budget model is in clarifying the importance of human impacts. Urban harbours are not in a 'natural' state, and increasing human activity, both locally and globally, affects their overall health. (C) 2010 Elsevier Ltd. All rights reserved.