Study of ephemeral rock pools on Domboshawa Mountain, Zimbabwe.

Abstract

Five ephemeral rock pools were sampled weekly from December 2006 to May 2007 on the Domboshawa Mountain. Physical variables, which included temperature, pH, dissolved oxygen, phosphorus, nitrogen, conductivity and pool morphometry (depth, total wetted surface and volume), were measured. The ecological variables measured were primary production, diversity and doubling time for the phytoplankton community. Geology and catchment characteristics affected variables such as nitrogen and conductivity. Pool morphometry is the main physical factor that structured these habitats. The pools were frequently flushed out. On a weekly average, the deepest pool (P1) was flushed out 73.61 times whilst the shallowest pool was flushed out 248.34 times. The shallowest pool (P3) had higher diurnal temperature variations than the deepest pool (P1). Both the minimum and maximum temperatures of 14.7oC and 33.6oC respectively were recorded in the shallowest pool (P3). Nevertheless, there was no significant difference ( , p > 0.05) in the ranges of temperatures experienced in the shallowest and the deepest pool. Deeper pools stratified whilst the shallow ones did not. Pool morphometry has an effect on doubling time in phytoplankton populations. In the deepest pool it took 19 days whilst in the shallowest pool it took 43 days for the Microcystis aeruginosa population to double. The larger pools were more diverse in species composition than the smaller pools. Temporal variation in the phytoplankton community was observed. The blue green algae, M. aeruginosa, increased in abundance towards the end of the rain season whilst the green algae, Spirogyra rhizobrachiales, decreased in abundance as the season progressed. Pool morphometry, catchment characteristics, temporal variation in temperature and the high weekly rates of flushing out were the main factors that affected these habitats.