Anthropogenic impacts on Waituna Lagoon: Reconstructing the environmental history

Abstract

Waituna Lagoon is a shallow temperate coastal lagoon in Southland, New Zealand. The 1,350 hectare Waituna Lagoon and associated wetland system is of national and international importance for its unique ecology and the cultural values placed upon it. Waituna is opened to the sea ca. 1-2 times annually, during which time it becomes estuarine. This opening regime has been artificially managed since 1908, with an unknown impact on the lagoon. Since 2007, Waituna appears to have experienced rapid degradation and eutrophication, possibly due to intensification of farming in its catchment. It has rare, extensive beds of the seagrasses Ruppia megacarpa and Ruppia polycarpa, which have been lost from many similar systems due to eutrophication and sedimentation. Waituna Lagoon is currently under active management to preserve its unique ecology, and its restoration to a more natural state is being considered. This study aims to reconstruct Waituna Lagoon’s natural opening regime and Ruppia dynamics, as understanding the natural ecosystem is important for restoration. The hypotheses are: (1) that under a natural opening regime, environmental variations within Waituna Lagoon were less frequent but more extreme; (2) that Ruppia was not present in Waituna Lagoon under a natural opening regime; and (3) that Ruppia was not dominant in Waituna Lagoon under a natural opening regime. Three push cores (130, 83 and 64 cm length) were collected from the sheltered Shand Bay and from the wind-exposed, deepest site in the main lagoon. Analyses of sedimentary material included organic and water content, sediment density and grain size, 210Pb, charcoal, indicator pollen, foraminifera and macrofossils. Images of the cores were taken via x-ray and the Geotek Multi-Sensor Core Logger. 210Pb dating of the sediment was unsuccessful, but the approximate date of transition to an anthropogenically modified system was inferred to occur at ca. 50 cm depth in Shand Bay using profiles of indicator pollen and charcoal. The historical opening regime was reconstructed using strata of elevated sand content to indicate open (marine) phases and strata of elevated organic content to indicate closed (freshwater) phases. Hypothesis one was supported. Under a natural opening regime phases of marine and terrestrial influence appeared to be more pronounced, and potentially lasted for longer periods of time. After ca. 1900 the sediment data indicated a higher frequency of phase changes inhibiting lagoon equilibration. An exception was the most recent sediments, which suggested a marine-influenced phase. Ruppia inhabited Waituna Lagoon under a natural opening regime, and circumstantial evidence suggests that it may have been more dominant at times under the natural opening regime. Hypotheses two and three were rejected. Ruppia pollen was more abundant when the lagoon was open, possibly due to increased light availability. Sediment chronologies are not consistent across Waituna Lagoon. Sediment characteristics differ between the deeper, wind- and tide-exposed mid-lake and shallow, sheltered Shand Bay sites. Sediment strata in the cores from the two sites could not be correlated, indicating that sediment dynamics at the two sites differed markedly. Sediment deposition in the lagoon is highly episodic and appears to derive from diverse sources, making it difficult to accurately reconstruct timelines related to changes in sedimentation and lagoon conditions.