Seaweed farming pressure affects seagrass and benthic macroalgae dynamics in Chwaka Bay (Zanzibar, Tanzania)

Agustín Moreira-Saporiti,Flower E Msuya,Mirta Teichberg,Dieuwke Hoeijmakers,Hauke Reuter

doi:10.1007/s10113-020-01742-2

Agustín Moreira-Saporiti, Flower E Msuya + Show 3 more

Open Access

https://doi.org/10.1007/s10113-020-01742-2

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Abstract

Farming of Eucheuma denticulatum is a major activity in Zanzibar affecting seagrass ecosystems primarily through shading and trampling. The aim of this study was to test the impacts of shading and trampling during seaweed farming on seagrass meadows composed by Halophila stipulacea and Thalassia hemprichii and their associated benthic macroalgae. Areas covered by these species were selected for the building of seaweed farms in three treatments: seaweed farm plots (with shading and trampling effects), trampling plots (with trampling effects only), and control plots (with no shading or trampling effects). Reduction of light within the plots was recorded over 9 weeks. Percentage cover of seagrasses and macroalgae and shoot density of seagrasses were measured over 12 weeks to assess the impact of shading and trampling by seaweed farming activities. Light was significantly reduced in the seaweed farm plots by 75 to 90% by the end of a seaweed growth cycle. H. stipulacea, despite its capacity for rapid growth, was significantly affected by the combination of shading and trampling under the seaweed farm treatment, while the climax seagrass species T. hemprichii was unaffected. Due to the decline in H. stipulacea, benthic macroalgae cover increased in the seaweed farm treatment, suggesting a change in seagrass community dynamics. In contrast, trampling had a negative effect on the benthic macroalgae as an isolated disturbance, which suggests that seagrasses are more resistant to trampling than macroalgae and would likely dominate the benthic macrophyte community under these conditions.

Highlights

Farming of red macroalgae of the genus Eucheuma is a major economic activity in Asian countries including the Philippines and Indonesia (Hurtado et al 2014)
We focused on the impacts of seaweed farms through their combined effects of shading and trampling, and on the impact of trampling alone as an isolated stressor
There was a small reduction in light outside the farmed plots, 15–25%, light reduction was significantly greater inside the seaweed farm plots at every sampling time, stabilizing between 75 and 90% from day 15 onwards (Fig. 2, Fig. 2 Light reduction (%) inside and outside the seaweed farms during the experiment

Summary

Introduction

Farming of red macroalgae of the genus Eucheuma is a major economic activity in Asian countries including the Philippines and Indonesia (Hurtado et al 2014). In 2016, the total production of Eucheuma worldwide was around 10,519 thousand tons of live weight (FAO 2018), of which 214 thousand tons were from the species Eucheuma denticulatum. The value of this seaweed comes from its carrageenan, a substance used for its gelling, thickening, emulsifying, and water holding properties (Juanich 1988). Several villages in Zanzibar adopted the farming of the red macroalgae

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