Effects of Sediment Accumulation on the Productivity of Habitat-Forming Seaweeds

Abstract

Increasing human population, coastal development, infrastructure and land use changes are causing large amounts of sediment to be transported into adjacent marine ecosystems. Macroalgae are often at the heart of these ecosystems, and despite the increasing interest in sedimentation around the world, there is little information on the effect that sediment has on subtidal macroalgal stands in New Zealand. This thesis investigates the effects of deposited sediment on two important habitat forming species of macroalgae in the Hauraki Gulf, Ecklonia radiata and Carpophyllum flexuosum. The first objective was to examine the temporal and spatial variability in the accumulation of sediment on these species and determine the primary environmental drivers responsible for this. This was examined by monitoring the amount of sediment on both species at a range of sites throughout 2011. The second objective of this thesis was to experimentally examine the effects of sediment accumulation on the growth and productivity of E. radiata and C. flexuosum. This was done by simulating the effects of sediment deposition on macroalgae for up to 11 days, removing the sediment and comparing the growth and photosynthetic rates of the sediment affected plants to controls. The amount of sediment accumulating on seaweeds was highly variable over time and among sites. At open coast sites dominated by E. radiata, sediment accumulation was negatively correlated with wave action, with sediment accumulating on seaweeds during periods of calm sea conditions. In contrast, at sheltered sites dominated by C. flexuosum, sediment accumulation was highly correlated with rainfall events. The maximum amount of sediment recorded on E. radiata was 4.5mg cm-2 which was estimated to reduce light levels by more than 75%. Not surprisingly sediment addition to E. radiata plants in the laboratory caused an immediate reduction in photosynthetic rates of and this effect was greatest at low irradiance levels. Addition of sediment to E. radiata for periods of up to 11 days caused plants to lose weight, and caused bleaching, rotting and perforations on the thalli. Subsequently this had detrimental effects on photosynthesis, considerably reducing oxygen production of the plant and reducing its ability to act as a fundamental provider at the base of the food chain. These effects were greatest at low light levels and suggest that sediment accumulation can have more of a longer-term detrimental effect on kelp production than just the effect of instantaneously reducing light levels and photosynthetic rates. Conversely, C. flexuosum was slightly more tolerant of sediment deposition with no evidence of bleaching, and had greater ability to acclimate to the effects of sediment accumulation. It is important we understand the consequences that human development resulting in increased sedimentation is having on natural marine systems. Evidence presented indicates that sediment deposition can reduce productivity of kelp systems, with a more extreme impact at low light levels. With impending climate change and increasing development set to increase sedimentation and reduce light levels in the coastal environment, this may further exacerbate the negative effects of sediment accumulation on kelp productivity. Therefore, there needs to be an emphasis on the importance of improved policies and land management practices to reduce terrestrially-derived sediment inputs into the coastal environment to ensure the maintenance of this important life-supporting habitat.