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Marine Ecology Progress Series

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MEPS 647:33-48 (2020)  -  DOI:

Shady business: the darkening of estuaries constrains benthic ecosystem function

Stephanie Mangan1,2,*, Karin R. Bryan1,2, Simon F. Thrush3, Rebecca V. Gladstone-Gallagher3, Andrew M. Lohrer4, Conrad A. Pilditch1,2

1School of Science, University of Waikato, Hamilton 3216, New Zealand
2Environmental Research Institute, University of Waikato, Hamilton 3216, New Zealand
3Institute of Marine Science, University of Auckland, Auckland 1142, New Zealand
4National Institute of Water and Atmospheric Research Ltd, Hamilton 3216, New Zealand
*Corresponding author:

ABSTRACT: Coastal intertidal soft-sediment habitats provide ecosystem services to millions of people worldwide, yet are under intense pressure from land-use change and sea-level rise (SLR). Both pressures interact to reduce light reaching the seafloor, thereby disrupting benthic primary producers and the ecosystem functions and services they provide. This study considers the implications of altered light climate on microphytobenthic (MPB) production in shallow estuaries. Continuous measurements of seafloor photosynthetically active radiation (PAR) were made over 9 mo on intertidal sandflats in 14 New Zealand estuaries spanning a turbidity gradient. A literature summary of benthic photosynthesis-irradiance curves was used to predict PAR limitation at sampling sites. Estimates of the proportion of time MPB would be light limited during emersion ranged from a median of 32-64% compared to a median of 55-100% during immersion. For estuaries close to 100% PAR limitation during immersion, emerged intertidal areas represent a refuge for MPB production which is vulnerable to SLR. Based on hypsometric curves (a representation of estuary bathymetry), the intertidal area of our study estuaries is predicted to decrease by 27-94% in response to SLR of 1.4 m. The combination of high PAR limitation during immersion and large losses of intertidal area will increase vulnerability to the loss of MPB production and the associated ecosystem services, which will push these ecosystems towards tipping points. The research highlights how the interplay between local and global scale stressors may ultimately trigger ecological collapse under future global change.

KEY WORDS: Light limitation · Photosynthesis-irradiance curves · Intertidal · Benthic · Sea-level rise · Hypsometry · Microphytobenthos · Estuary

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Cite this article as: Mangan S, Bryan KR, Thrush SF, Gladstone-Gallagher RV, Lohrer AM, Pilditch CA (2020) Shady business: the darkening of estuaries constrains benthic ecosystem function. Mar Ecol Prog Ser 647:33-48.

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