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Primary producers as sinks for nitrogen and phosphorus in the Great Brak Estuary
Expanded Title:Temporarily Open/Closed Estuaries generally have low river inflow, a long water residence time due to weak flushing, and prolonged mouth closure (Whitfield 1992, Taljaard et al. 2009 b), which makes this type of estuary vulnerable to nutrient enrichment and the accumulation of organic matter (Newton and Mudge 2005, Human and Adams 2011). The Great Brak Estuary is located on the south coast of South Africa and is 6.2 km in length. A dam built just 3 km upstream from the estuary has severely affected the natural flow to the estuary and has reduced the amount of freshwater flow to the estuary by as much as 56%. This in turn has led to reduced flushing, accumulated organic matter and degradation in the water quality. In the closed mouth state in the Great Brak, it is unlikely that nutrients in river water will reach the lower end of the system. It is therefore expected that in the lower reaches the nutrients will be supplied from the benthos. This is the first detailed study that accounts for the nutrient storage in macrophytes by using actual measured data. The relative importance of the submerged macrophytes and macrolagae to other sources was quantified in the nutrient budget and it was shown that they play an important role in nutrient storage and subsequent cycling. It has also shown that, in the closed mouth state, the benthos becomes the major source of nutrients to the water column and supplies a similar percentage of N and P to that which gets stored in the submerged macrophytes and macroalgae. The submerged macrophytes and macroalgae took up to 30% TN and 38% TP from the water column during the closed phase. However, the sediments were the highest contributors of TN and TP to the system and contributed about 30% of the TN and 40% TP toward the nutrient budget. When the mouth opened under strong flow conditions the Great Brak Estuary acted as a source of nutrients to the sea. Of the three estuary states (freshwater dominated, marine dominated and closed mouth) occurring in TOCEs as described by Snow and Taljaard (2007), the closed phase represents a relatively stable state. However, low flow conditions and mouth closure can support the bottom-up control of primary producers; i.e. greater light availability, limited nutrients in water column, more stable environment - conditions that are likely to support the growth of opportunistic filamentous macroalgae. The C.A.P.E. Estuaries Program is in the process of developing an Estuary Management Plan for the Great Brak Estuary. The information gained through this study highlights the sources of nutrients in the estuary and proposes management plans to mitigate the effects.
Date Published:01/01/2014
Document Type:Research Report
Document Subjects:Ecosystem - Biodiversity protection
Document Keywords:Nutrient Management, Water Quality
Document Format:Report
Document File Type:pdf
Research Report Type:Standard
WRC Report No:1982/1/14
ISBN No:978-1-4312-0493-9
Authors:Snow GC; Human LRD
Project No:K5/1920
Originator:WRC
Organizations:Nelson Mandela Bay Municipality
Document Size:2 824 KB
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