Fish grazing enhanced by nutrient enrichment may limit invasive seagrass expansion

Fee O.H Smulders,S Tatiana Becker,Justin E Campbell,Elisabeth S Bakker,Mickey J Boässon,Mark M Bouwmeester,J Arie Vonk,Marjolijn J.A Christianen

doi:10.1016/j.aquabot.2021.103464

Abstract

The success of invasive macrophytes can depend on local nutrient availability and consumer pressure, which may interact. We therefore experimentally investigated the interacting effects of nutrient (nitrogen and phosphorus) addition, the exclusion of large herbivorous fishes and mimicked grazing on the expansion rates of the invasive seagrass Halophila stipulacea. The experiments were established on Bonaire and Aruba, two islands in the southern Caribbean, which differ in fish community structure. We observed that multiple Caribbean fish species feed on H. stipulacea. At both study sites, nutrient enrichment decreased invasive leaf carbon:nitrogen ratios. However only on Bonaire, where herbivore fish abundance was 7 times higher and diversity was 4.5 times higher, did nutrient enrichment result in a significant reduction of H. stipulacea expansion into native Thalassia testudinum meadows. This effect was likely due to increased herbivory on nutrient enriched seagrass leaves, as we found that excluding large herbivorous fish (e.g. parrotfish) doubled invasive expansion rates in bare patches on Bonaire. On Aruba, H. stipulacea expansion rates were higher overall, which coincided with lower abundances and diversity of native fishes, and were limited by mimicked fish grazing. We suggest that top-down control by the native fish community may counteract eutrophication effects by increased grazing pressure on nutrient-rich invasive seagrass leaves. We conclude that diverse and abundant herbivore communities likely play an important role in limiting invasion success and their conservation and restoration may serve as a tool to slow down seagrass invasions.

Highlights

Coastal ecosystems around the world are experiencing a rapid in crease in the number of non-native species that often negatively impact ecosystem functioning (Gallardo et al, 2016; Molnar et al, 2008; Papacostas et al, 2017)
We suggest that top-down control by the native fish community may counteract eutrophication effects by increased grazing pressure on nutrient-rich invasive seagrass leaves
We address the following questions: (1) Does nutrient enrichment impact H. stipulacea leaf nutrient content and expansion rates? (2) Does fish grazing affect the expansion rate of H. stipulacea? (3) How do nutrient enrichment and fish grazing interact to impact the expansion rates of invasive H. stipulacea? To answer these questions, we set up experiments at two islands in the southern Caribbean that likely differed in their top-down strength due to contrasting protection status: a protected seagrass meadow on Bonaire and an unprotected meadow on Aruba

Summary

Introduction

Coastal ecosystems around the world are experiencing a rapid in crease in the number of non-native species that often negatively impact ecosystem functioning (Gallardo et al, 2016; Molnar et al, 2008; Papacostas et al, 2017). The success of invasive macropyhtes may depend on local conditions such as nutrient availability and native biodiversity (Chapman, 2017; Havel et al, 2015). Preference of native herbivores for invasive macrophytes can be site and species specific (Morrison and Hay, 2011; Tomas et al, 2010). Since most marine herbivores are considered generalists (Fleming and Dibble, 2014; Parker and Hay, 2005), there are various examples of native generalist herbi vores reducing invasive macrophyte success Since most marine herbivores are considered generalists (Fleming and Dibble, 2014; Parker and Hay, 2005), there are various examples of native generalist herbi vores reducing invasive macrophyte success (e.g. Ribas et al, 2017; Tomas et al, 2011)

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