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Contribution of seagrass (Zostera muelleri) to estuarine food webs revealed by carbon and nitrogen stable isotope analysis

Seagrass is a conspicuous marine angiosperm that forms extensive beds in coastal and intertidal estuarine areas. In many regions worldwide seagrass is in decline and there is a pressing need to understand the function and importance of seagrasses to coastal ecosystems. To assess the importance of seagrass (Zostera muelleri) to secondary production, a dual carbon and nitrogen stable isotope study was conducted using potential food sources (seagrass live blades and detritus, phytoplankton and microphytobenthos) and selected macro-invertebrate consumers at four study sites in three estuaries (Raglan, Tauranga and Whangapoua Harbours). Sampling focused on bivalves, gastropods, crustaceans and annelid polychaetes, representing a range of functional feedings groups (suspension- and deposit-feeders) that were found in seagrass beds and adjacent non-vegetated sediment. In addition, macro-invertebrate community analysis determined the abundances of species selected for isotope analysis and their relative importance to the macro-invertebrate communities. Macro-invertebrate species analysed for stable isotope analysis accounted for between 26 - 63% and 37 - 95% of the macro-invertebrate abundance within seagrass and nonvegetated sediment, respectively. The distinctive δ¹³ C and δ¹⁵N isotope signatures of Z. muelleri (δ¹³C = -10 and δ¹⁵N = 6%₀), phytoplankton (-23 and 7%₀) and microphytobenthos (-20 and 5%₀) allowed these potential food sources to be traced through the food webs studied. The herbivorous gastropods Diloma subrostrata and Zeacumantus lutulentus had similar δ¹³C values to seagrass (mean δ¹³C = -10%₀) and the δ¹³C values of the deposit feeding bivalve Macomona liliana, crustacean Helice crassa and annelid polychaete species Orbinia papillosa and Aquilaspio aucklandica were generally intermediate between microphytobenthos and Z. muelleri. Austrovenus stutchburyi (suspension feeding bivalve) was depleted in ¹³C and had a mean δ¹³C value similar to microphytobenthos. Polychaetes Aglaophamus macroura, Aonides oxycephala and Glycera americana consistently recorded δ¹³ C values similar to the other consumers (excluding A. stutchburyi) but had δ¹⁵N values characteristic of a higher trophic level and a predatory existence. Differences in the isotopic ratios of macro-invertebrate species between sampling locations within seagrass beds and adjacent non-vegetated sediment were not significant, indicating that seagrass contributes to the food chain in nearby (< 20 m) non-vegetated sediment. Significant differences in isotopic signatures existed between east and west coast sites, with food sources and benthic consumers consistently displaying enriched δ¹⁵N and depleted δ¹³C values at Raglan on the west coast suggesting differences in the inorganic nutrient inputs fuelling primary production. Results suggest that secondary production was fuelled by benthic (seagrass and microphytobenthos) rather than pelagic organic matter (phytoplankton). Z. muelleri was an exclusive food source for gastropods D. subrostrata and Z lutulentus and also contributed to the diets of M. liliana, H. crassa, O. papillosa and A. aucklandica. Spatial differences occurred between study sites but not between sampling locations. This study confirms that Z. muelleri plays a role in the production of benthic macro-invertebrate communities and its loss from estuarine ecosystems will impact negatively on secondary production.
Type of thesis
Hailes, S. (2006). Contribution of seagrass (Zostera muelleri) to estuarine food webs revealed by carbon and nitrogen stable isotope analysis (Thesis, Master of Science (MSc)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/12618
The University of Waikato
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