Ecosystem function responses to nutrient enrichment mediated by mud content in soft sediment habitats

ABSTRACT

Estuaries are threatened with increasing sedimentation and nutrients, however research is limited on the interaction of these stressors on ecosystem functions. A field experiment adding nitrogen (N) fertiliser at high (600 g N m⁻²) and medium (150 g N m⁻²) concentrations and control plots was established in two sandy and two muddy sites (mud content 0–15%) to investigate context dependency of nitrogen enrichment on nutrient cycling and primary production. There was a significant interaction of mud x treatment on ammonium (NH₄⁺) fluxes. High N-addition plots had 133x greater dark NH₄⁺ efflux than controls in muddy sediments, compared to 17x greater dark NH₄⁺ efflux in sandy sediments. Net primary production increased with mud content, while photosynthetic efficiency of the microphytobenthos (GPP_chl-a) decreased above 4% mud. Mud content also affected the bivalve, Austrovenus stutchburyi (>30 mm), macrofaunal abundance and functional groups, and their influence on nutrient cycling. Thus minor increases in mud can alter the ecosystem response to nutrient enrichment directly through changes in sediment properties and indirectly by affecting the bivalves and macrofauna that influence these processes. This study highlights how multiple stressors interact to predispose estuaries to tipping points.

KEYWORDS:

• Nutrient regeneration
• primary production
• ecosystem function
• land-derived threats
• nutrient enrichment
• sedimentation
• Austrovenus stutchburyi

Acknowledgements

We would like to thank Jack Hall, Sorrel O’Connell-Milne and Stina Kolodzey involved in the field sampling and laboratory work. Linda Groenewegen is thanked for running the nutrient analyses at Portobello Marine Laboratory. An anonymous reviewer and the handling editor are thanked for valuable feedback that helped improve the manuscript. ST, CS, CAP and SFT have designed the study and ideas. ST has performed the laboratory work and analysis. ST and CS undertook the field work. ST wrote the manuscript and compiled the data with assistance from CS and input from CAP, SFT.

Disclosure statement

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

Additional information

Funding

This work was funded by The New Zealand National Science Challenge Sustainable Seas, Dynamic Seas, Tipping Points project (CO1X1515), Environment Southland, and a scholarship from the New Zealand Coastal Society to ST.