Estuaries as coastal reactors: importance of shallow seafloor habitats for primary productivity and nutrient transformation, and impacts of sea level rise

ABSTRACT

Estuaries are hotspots of primary productivity and nutrient transformation that contribute to food webs and ecosystem functioning locally and in adjacent ecosystems. The depth-dependence of nutrient transformation and primary productivity rates in the water column and the seafloor were investigated, and the lateral transport of solutes and materials across the estuary mouth were quantified. Using an estuary dominated by shallow soft-sediment habitats as a case study, the effects of sea level rise (SLR) on productivity and nutrient transformation processes were projected. The estuary was a net importer of dissolved nutrients from the coast, and a net exporter of suspended sediments and chlorophyll a, supporting the notion that estuaries are important nutrient transformation reactors. A significant depth (and light) effect on productivity indicates that increasing stressors associated with climate change that reduce light at the seafloor (SLR and increased turbidity) will negatively impact estuarine productivity. Intertidal and shallow subtidal benthic habitats were responsible for most of the productivity of the estuary and this is likely to be consistent for other shallow estuaries globally. SLR and anthropogenic intervention that prevents landward migration (i.e. seawalls and armouring), will result in the loss of these habitats and their significant contributions to adjacent coastal ecosystems.

KEYWORDS:

• Nutrient flux
• microphytobenthos
• benthic productivity
• estuarine subsidy
• hypsometry
• coastal armouring

Acknowledgements

Thanks to divers and technicians who contributed to field work and laboratory processing, including Katie Cartner, Sam Parkes, David Bremner, Rod Budd, Julia Simpson, Ivan Rodil and Glen Reeve. We also thank Judi Hewitt for helpful comments on earlier drafts of the manuscript. We appreciate the invitation to contribute to this Special Issue, Wahapū: Transcending boundaries in Aotearoa New Zealand estuaries, and we thank the editorial team and two anonymous reviewers for their comments.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was funded by NIWA under the Coasts and Oceans Research Programme 5 [SCI 2019/20] and progenitor programmes.