First insights into the impacts of benthic cyanobacterial mats on fish herbivory functions on a nearshore coral reef

Abstract

Benthic cyanobacterial mats (BCMs) are becoming increasingly common on coral reefs. In Fiji, blooms generally occur in nearshore areas during warm months but some are starting to prevail through cold months. Many fundamental knowledge gaps about BCM proliferation remain, including their composition and how they influence reef processes. This study examined a seasonal BCM bloom occurring in a 17-year-old no-take inshore reef area in Fiji. Surveys quantified the coverage of various BCM-types and estimated the biomass of key herbivorous fish functional groups. Using remote video observations, we compared fish herbivory (bite rates) on substrate covered primarily by BCMs (> 50%) to substrate lacking BCMs (< 10%) and looked for indications of fish (opportunistically) consuming BCMs. Samples of different BCM-types were analysed by microscopy and next-generation amplicon sequencing (16S rRNA). In total, BCMs covered 51 ± 4% (mean ± s.e.m) of the benthos. Herbivorous fish biomass was relatively high (212 ± 36 kg/ha) with good representation across functional groups. Bite rates were significantly reduced on BCM-dominated substratum, and no fish were unambiguously observed consuming BCMs. Seven different BCM-types were identified, with most containing a complex consortium of cyanobacteria. These results provide insight into BCM composition and impacts on inshore Pacific reefs.

Benthic Cyanobacterial Diversity and Antagonistic Interactions in Abrolhos Bank: Allelopathy, Susceptibility to Herbivory, and Toxicity

“…The limited sample size of our feeding trials requires caution in interpreting such results and making generalizations of this pattern to BCMs in general. Given that a significant discrepancy between feeding selectivity and nutrient content of cyanobacteria was reported for reef herbivores (Mendes et al, 2018), BCMs consumption and deterrence seem to be a trade-off between its composition, growth form, and chemistry (Cissell and McCoy, 2021;Ford et al, 2021). The bottom-up effects on reef herbivores remain elusive as contrasting evidence has recently emerged (Cissell et al, 2019;Ford et al, 2021).…”

“…Given that a significant discrepancy between feeding selectivity and nutrient content of cyanobacteria was reported for reef herbivores (Mendes et al, 2018), BCMs consumption and deterrence seem to be a trade-off between its composition, growth form, and chemistry (Cissell and McCoy, 2021;Ford et al, 2021). The bottom-up effects on reef herbivores remain elusive as contrasting evidence has recently emerged (Cissell et al, 2019;Ford et al, 2021). Consumers with different feeding strategies and nutritional demands may then target high nutritious cyanobacteria (Clements and Choat, 2018), and also associated non-cyanobacterial components (Cissell and McCoy, 2021), or avoid less nutritious (Nagarkar et al, 2004) and toxic cyanobacteria (Ford et al, 2021).…”

“…The bottom-up effects on reef herbivores remain elusive as contrasting evidence has recently emerged (Cissell et al, 2019;Ford et al, 2021). Consumers with different feeding strategies and nutritional demands may then target high nutritious cyanobacteria (Clements and Choat, 2018), and also associated non-cyanobacterial components (Cissell and McCoy, 2021), or avoid less nutritious (Nagarkar et al, 2004) and toxic cyanobacteria (Ford et al, 2021).…”

“…Phase shifts from coral dominated to opportunistic BCMs taxa have been reported worldwide (de Bakker et al, 2017;Ford et al, 2018), and such expansion disturbs reef trophic dynamics (Ford et al, 2021) by reducing energy flow and ultimately promotes a trophic simplification of the ecosystem (Ullah et al, 2018). This trophic disruption brings reef communities to alternative unstable states (Teixeira et al, 2021), because cyanobacteria are highly efficient in nutrient uptake and grow fast (den Haan et al, 2016;Ford et al, 2018), boosting their capacity to outcompete other benthic species and change community structure (e.g., de Bakker et al, 2017;Ribeiro et al, 2018;Teixeira et al, 2021).…”

A new threat to local marine biodiversity: filamentous mats proliferating at mesophotic depths off Rapa Nui

“…These filamentous mats could develop into such dense blooms that they could even wash ashore, producing a mass accumulation, as reported by Nagle & Paul (1999) for Guam. At this location, benthic marine cyanobacterial blooms often occur in the presence of diverse assemblages of herbivorous fishes and urchins, but the underlying factors causing these proliferations, as well as the interaction mechanisms between grazers and these mats (since cyanobacteria are known to produce feeding-deterrent compounds), are still poorly understood ( Cissell, Manning & McCoy, 2019 ; Ford et al, 2021 ). In addition, cyanobacteria have been directly linked with ciguatera fish poisoning outbreaks ( Laurent et al, 2008 ), and mats can create suitable habitats for other toxic microalgae, including toxin-producing dinoflagellates, thus generating co-occurring blooms ( Paerl & Otten, 2013 ).…”