Diversity in phytoplankton communities: A field test of the intermediate disturbance hypothesis

Abstract

The species dynamics is the feature of the ecosystem succession and is a key issue in the disturbance ecology. The intermediate disturbance hypothesis proposes that the highest diversity is maintained at intermediate scales of disturbance. This hypothesis has been applied to phytoplankton in water-body. It was found out that a low similarity between consecutive water samples is the result of a drastic disturbance, whereas a high similarity indicates a stable condition. To investigate whether the intermediate disturbance hypothesis is applicable to phytoplankton communities in water-body with a pollution gradient, we provided a field test in Changguangxi National Urban Wetland Park in Wuxi, China. We sampled once a month over a span of four seasons. The phytoplankton species were identified, and water quality monitoring was conducted as the disturbance factor in this study, which included the carbon, nitrogen and phosphorus concentrations. We calculated the similarities of phytoplankton species and water quality among the sampling locations and regarded them as the indicators of disturbance to study the impacts of water pollution. Our results indicated that: (1) the fitting curves of phytoplankton community diversity and similarity showed unimodal patterns over four seasons, which supported the intermediate disturbance hypothesis; (2) the response of phytoplankton diversity to water quality levels was not clear. When the water quality similarity was plotted against the phytoplankton diversity, there was no unimodal relationship, neither in the particular season nor over an entire year. We concluded that in this study the intermediate disturbance hypothesis was confirmed by the phytoplankton composition similarity, but water quality failed to be an indicator of a disturbance.