The comparative limnology of two reservoirs in the Australian wet/dry tropics
Creators: name: Townsend, Simon
Publisher: name: Charles Darwin University
Year: 1997

description: Darwin River Reservoir (DRR) and Manton River Reservoir (MRR) are two mature (age &gt;25 years}, shallow (average depth approx. 4 m) water bodies in the Australian wet/dry tropics (l3°S, 131 °E). The reservoirs lie in adjacent catchments, 6km apart, and are exposed to the same regional climate. Both reservoirs drain undisturbed catchments of similar geology and vegetation, but differ in their retention time (DRR average 2.3 years, MRR average 0.5 years) and wind exposure. Average wind speeds over DRR were three times greater than those over MRR. This study, based on an eight year data set, examined the affect of climate, retention time and wind exposure on the limnology of the two reservoirs. The major findings of the study are as follows: • The colour of each reservoir, a surrogate measure of the concentration of dissolved humic substances and the main determinant of light penetration in the reservoirs, was dependent on annual retention time. • Similar to other tropical water bodies, the seasonal pattern of temperature and heat content in the reservoirs was controlled principally by net radiation (mainly short-wave radiation) and evaporative energy fluxes. • In most years, both reservoirs experienced two annual temperature and heat content minima and maxima. A single minimum and maximum was experienced when monsoon weather activity was brief, lasting just a few days or less. This contrasts with temperate water bodies, but is similar to most water bodies in the wet/dry tropics. • Although subject to the same regional climate, the mixing regimes of the study reservoirs differed. DRR is classified discontinuous polymictic, whilst MRR is classified monomictic. These differences are attributed to the different retention time and wind exposure of the reservoirs, and emphasise the role of local geographic and reservoir specific influences on mixis relative to regional climatic influences. • Both reservoirs experience long periods of thermal stratification with rates of hypolimnnetic deoxygenation greater than those measured in temperate water bodies. Such high rates were attributed principally to each reservoirs' warm hypolimnetic temperatures, whilst reservoir morphometry and trophic state were considered secondary. The use of hypolimnetic deoxygenation as an index of lake trophic status is not globally applicable due to its temperature dependence. • On an annual time scale, phytoplankton biomass of both reservoirs is phosphorus limited. This conclusion is reached based on an empirical relationship between average total phosphorus and chlorophyll a concentrations, and high total nitrogen to total phosphorus concentration ratios. During the wet season, wash-out in MRR is also likely to limit phytoplankton biomass. • The phosphorus-chlorophyll a relationship for the reservoirs predicted a higher average chlorophyll a concentration, for a total phosphorus concentrations of 10 µg 1(-1)', compared to predictions based on temperate data sets. This difference was statistically significant for only two of the four temperate predictions. No conclusive evidence of a higher biomass of phytoplankton, per unit of limiting nutrient, was found. • The phytoplankton composition of the two reservoirs was dominated by an assemblage of Peridinium, Botryococcus braunii and Desmidaceae, though the relative dominance of each varied between the reservoirs. These populations were maintained during holomixis, which generally lasted a few days. Some holomictic events favoured the growth of Bacillariophyta (mainly Aulacoserira granulata), which were occasionally dominant in MRR. Unlike many higher latitude and some tropical water bodies, there was no evidence of an autogenic succession, from r-strategist to K-strategist phytoplankton, during thermal stratification. Both reservoirs were dominated by K-strategist phytoplankton throughout the year. • The study concludes the regional climate of a water body, whilst being an important determinant of reservoir limnology, is not the only influence. Other influences, specific to the water body such as retention time and wind-exposure, also have a profound influence. Such influences are probably more pronounced in the wet/dry tropics, due to the low stability of stratification of tropical water bodies compared with their higher latitude counterparts.