New Zealand Journal of Marine and Freshwater Research abstracts
River inputs, re-mineralisation and the spatial and temporal distribution
of inorganic nutrients in Tasman Bay, New Zealand
Lincoln MacKenzie
Cawthron Institute
Private Bag 2
Nelson, New Zealand
email: Lincoln.Mackenzie@cawthron.org.nz
Abstract To improve understanding of the temporal and
spatial variability of biological productivity relevant to fisheries enhancement
within Tasman Bay, New Zealand, it is necessary to evaluate the importance
of variations in river inflows on inorganic nutrient dynamics. To achieve
this, a series of synoptic surveys of surface and water column salinity fields
and inorganic nutrient concentrations were carried out over a 1-year period,
at seasonally representative times, during, and soon after, a range of flow
regimes in the major freshwater tributaries of Tasman Bay. An assessment
was made of the relative importance of external (i.e., river borne) and internal
(i.e., recycling) nutrient sources that control the fertility of the Tasman
Bay water column. Flood events in the bay’s catchment cause significant changes
in the levels of dissolved nitrogen, phosphorus, and silica in surface waters,
and on an annual basis 60-70% of the input of “new” dissolved inorganic nitrogen
from freshwater tributaries is contributed by the Motueka River. As a consequence
there is a pronounced east to west gradient in nutrient enrichment, and oceanic
water most frequently intrudes along the eastern shore. Surface salinity
and inorganic nutrient distribution patterns and inferred water movements
are consistent with the left hand deflection of buoyant river plumes. The
predictable timing and magnitude of the winter nitrate/nitrite maxima suggests
that light is the main factor limiting phytoplankton production and nutrient
assimilation during the winter, and that benthic and pelagic re-mineralisation
processes are the dominant mechanism generating the annual nutrient maxima
within the water column as a whole. Calculations suggest that the net accumulation
of inorganic nitrogen via endogenous re-mineralisation over the autumn-winter
period is of a similar order of magnitude to the annual introduction of new
nitrogen from freshwater sources. How this compares to inputs of new nitrogen
by advection from oceanic sources has yet to be determined.
Keywords Tasman Bay; inorganic nutrients; river inputs;
re-mineralisation
M03065; Received 3 October 2003; accepted 23 April 2004; Online publication
date 5 August 2004
New Zealand Journal of Marine and Freshwater Research, 2004, Vol. 38:
681-704
0028-8330/04/3804-0681 © The Royal Society of New Zealand 2004
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