Human impacts in an urban port: The carbonate budget, Otago Harbour, New Zealand

Otago Harbour is a long (23 km), narrow (mean width = 2 km), shallow (mean water depth = 4.5 m) tidal inlet covering 46 km2 on the southeast coast of South Island, New Zealand (45°50′S, 170°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 (∼10,000 tonnes CaCO3 y−1) and by transport though the harbour entrance from the longshore system (∼24,000 tonnes CaCO3 y−1). Shellfishing (∼2 tonnes CaCO3 y−1), dredging (∼18,000 tonnes CaCO3 y−1), and early sea-floor processes such as abrasion and dissolution (∼2000 tonnes CaCO3 y−1) remove carbonate from the system. The present-day carbonate budget results in ∼14,000 tonnes CaCO3 y−1 sediment storage, equivalent 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.