Nutrient History Affects the Response and Resilience of the Tropical Seagrass Halophila stipulacea to Further Enrichment in Its Native Habitat.

Stephanie B Helber,Michael Schmid,Marleen Stuhr,Gidon Winters,Stefanie Bröhl,Mirta Teichberg,E F Belshe,Hauke Reuter

doi:10.3389/fpls.2021.678341

Abstract

Eutrophication is one of the main threats to seagrass meadows, but there is limited knowledge on the interactive effects of nutrients under a changing climate, particularly for tropical seagrass species. This study aimed to detect the onset of stress in the tropical seagrass, Halophila stipulacea, by investigating the effect of in situ nutrient addition during an unusually warm summer over a 6-month period. We measured a suite of different morphological and biochemical community metrics and individual plant traits from two different sites with contrasting levels of eutrophication history before and after in situ fertilization in the Gulf of Aqaba. Nutrient stress combined with summer temperatures that surpassed the threshold for optimal growth negatively affected seagrass plants from South Beach (SB), an oligotrophic marine protected area, while H. stipulacea populations from North Beach (NB), a eutrophic and anthropogenically impacted area, benefited from the additional nutrient input. Lower aboveground (AG) and belowground (BG) biomass, reduced Leaf Area Index (LAI), smaller internodal distances, high sexual reproductive effort and the increasing occurrence of apical shoots in seagrasses from SB sites indicated that the plants were under stress and not growing under optimal conditions. Moreover, AG and BG biomass and internodal distances decreased further with the addition of fertilizer in SB sites. Results presented here highlight the fact that H. stipulacea is one of the most tolerant and plastic seagrass species. Our study further demonstrates that the effects of fertilization differ significantly between meadows that are growing exposed to different levels of anthropogenic pressures. Thus, the meadow’s “history” affects it resilience and response to further stress. Our results suggest that monitoring efforts on H. stipulacea populations in its native range should focus especially on carbohydrate reserves in leaves and rhizomes, LAI, internodal length and percentage of apical shoots as suitable warning indicators for nutrient stress in this seagrass species to minimize future impacts on these valuable ecosystems.

Highlights

Seagrass meadows are becoming increasingly threatened, as their distribution along coastal areas makes them especially vulnerable toward local anthropogenic pressures (Orth et al, 2006; Wilkinson and Salvat, 2012)
The response of H. stipulacea populations to thermal and nutrient stress were compared between two sites that are already heavily exposed to anthropogenic pressures (North Beach – NB) and two sites in a marine protected area with relatively low anthropogenic pressures (South Beach – SB) along the western coast of the northern Gulf of Aqaba (Eilat, Israel; Figure 1)
Our experimental design investigating changes in morphological, biochemical, structural and population level traits proved useful for the identification of indicators for nutrient stress in the tropical seagrass H. stipulacea in its native range

Summary

Introduction

Seagrass meadows are becoming increasingly threatened, as their distribution along coastal areas makes them especially vulnerable toward local anthropogenic pressures (Orth et al, 2006; Wilkinson and Salvat, 2012). In addition to being vital for adjacent ecosystems (e.g., coral reefs), many of these services and functions have important impacts on local economies and human health. Halophila stipulacea (Forsskål) Ascherson is a tropical seagrass species native to the Indian Ocean, Red Sea and Persian Gulf (Lipkin, 1975) It invaded the eastern Mediterranean through the Suez Canal some 150 years ago (Lipkin, 1975; Sghaier et al, 2011), but has expanded into the Caribbean, where it outcompetes the native seagrass species Syringodium filiforme, Halodule wrightii, and Halophila decipiens (Ruiz and Ballantine, 2004; Willette and Ambrose, 2012; Willette et al, 2014; Smulders et al, 2017; Winters et al, 2020). The ability of H. stipulacea to rapidly acclimate to a wide range of environmental conditions paired with its fast growth rates, leaf turnover and high fragment dispersal are possible reasons for its worldwide colonization success and, sparked interests in its population dynamics as well as its resilience toward anthropogenic pressures (Weatherall et al, 2016; O’Brien et al, 2018b)

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