Eutrophication of a small, deep lake in southern New Zealand: the effects of twentieth-century forest clearance, changing nutrient influx, light penetration and bird behaviour

This study provides a high resolution multi-proxy record of the response of an aquatic ecosystem (Alexander Lake) to forest clearance in New Zealand in the late twentieth century (ca. 1950–2006 AD). New chironomid-based transfer functions for lake water total nitrogen (TN) concentration were applied to the Alexander Lake chironomid record. A test of the significance of reconstructions based on multiple model types indicates that a model with the highest r² and lowest root mean squared error of prediction may not necessarily perform the best when applied to a particular site. The chironomid-based TN reconstruction and other proxies suggest a complex response by a stained water (dystrophic) lake in a forested catchment to deforestation. Minor perturbations and nutrient influx may favour increased phytoplankton production, but continued light attenuation by dissolved organic carbon and humic compounds prevents proliferation of submerged macrophytes. Complete mechanical forest clearance resulted in a short term pulse of nutrients and eutrophication. The long term effect of deforestation was to increase light penetration and favour the growth of submerged macrophytes. Continued eutrophication of Alexander Lake could be due to a contribution of bird-derived nutrients. Deforestation around Alexander Lake has created a perfect moulting site for Paradise Shelducks (Tadorna variegata Gmelin). The input of total phosphorus from T. variegata could be enough to trigger blooms of Microcystis that currently occur in the lake. Changes in bird behaviour in response to changes in vegetation should therefore be considered a possible result of past (including prehistoric) and future deforestation in New Zealand.