Relationships between water level fluctuations and vegetation diversity in shallow water of New Zealand lakes

Abstract

Hydrological and vegetation data from 21 New Zealand lakes were used in the study, to assess how water level fluctuations affected the diversity of a low-growing species-rich littoral plant community (here LMC). Species diversity of the LMC was affected by the water level range, the frequency of variation and the duration of low water level events. Diversity tended to increase with monthly level range, with a possible saturation at an inter-quartile range (the monthly water level fluctuation occurring between 25 and 75% of the time) of approximately 1 m. The species richness was much lower in lakes with inter-annual level variations than in lakes with intra-annual fluctuations. This indicates a relative long establishment time for a species-rich LMC. Up to a period of 1 month, increasing duration of events below median water level appeared to increase species diversity, though events longer than 2 months reduced it. To determine what hydrological conditions determine the upper depth limit of the LMC we made a series of paired sample tests of the measured upper limit of the LMC and the water levels where the mean length of dry period is 10, 30, 60, 120 and 180 days. We found that the observed upper limit best corresponded to the level where the dry period lasted between 10 and 30 days. Hence, the community was not observed where aerial exposure exceeds 30 days. Amongst the species found in the LMC, Elatine gratioloides, Glossostigma sp., Lilaeopsis ruthiana and Limosella lineata are the commonest species under most conditions, and can be considered as the most competitive species in the community. A generalised optimum habitat for a diverse LMC can thus be defined as occurring in a 1 m monthly water level range, with a mean duration of low level events lasting up to 1 month in lakes with low inter-annual water level fluctuation.