Organic matter degradation drives benthic cyanobacterial mat abundance on Caribbean coral reefs.

Hannah J Brocke,Hannah J Brocke,Hannah J Brocke,Hannah J Brocke,Hannah J Brocke,Miriam Weber,Lubos Polerecky,Joachim Claudet,Joachim Claudet,Joachim Claudet,Dirk De Beer,Maggy M Nugues,Maggy M Nugues,Maggy M Nugues,Maggy M Nugues,Chaolun Allen Chen

doi:10.1371/journal.pone.0125445

Abstract

Benthic cyanobacterial mats (BCMs) are impacting coral reefs worldwide. However, the factors and mechanisms driving their proliferation are unclear. We conducted a multi-year survey around the Caribbean island of Curaçao, which revealed highest BCM abundance on sheltered reefs close to urbanised areas. Reefs with high BCM abundance were also characterised by high benthic cover of macroalgae and low cover of corals. Nutrient concentrations in the water-column were consistently low, but markedly increased just above substrata (both sandy and hard) covered with BCMs. This was true for sites with both high and low BCM coverage, suggesting that BCM growth is stimulated by a localised, substrate-linked release of nutrients from the microbial degradation of organic matter. This hypothesis was supported by a higher organic content in sediments on reefs with high BCM coverage, and by an in situ experiment which showed that BCMs grew within days on sediments enriched with organic matter (Spirulina). We propose that nutrient runoff from urbanised areas stimulates phototrophic blooms and enhances organic matter concentrations on the reef. This organic matter is transported by currents and settles on the seabed at sites with low hydrodynamics. Subsequently, nutrients released from the organic matter degradation fuel the growth of BCMs. Improved management of nutrients generated on land should lower organic loading of sediments and other benthos (e.g. turf and macroalgae) to reduce BCM proliferation on coral reefs.

Highlights

Cyanobacteria are ubiquitous on coral reefs and play an important role in reef formation and nutrient cycling [1]
benthic cyanobacterial mats (BCMs) growth on coral reefs is typically limited by nutrients [6,24,25,26,27]
BCMs were congruent with high PO43- and NOx in their close vicinity in an otherwise oligotrophic water column (NOx ~0.5 μM; PO43- ~0.1 μM), regardless of depth and BCM dominance on an island scale

Summary

Introduction

Cyanobacteria are ubiquitous on coral reefs and play an important role in reef formation and nutrient cycling [1]. On declining reefs, they can form dense and widespread benthic cyanobacterial mats (BCMs), with negative consequences for reef health [1,2]. The mats reduce coral settlement and recruitment [12], alter coral-associated microbial communities [13], act as coral pathogens [14], and produce chemicals which have been linked to mass reef fish die-offs and deter grazing [15,16]. As many cyanobacteria are able to fix nitrogen (N) [17], their proliferation could increase fixed nitrogen in the system, which may enhance the growth of coral competitors, such as macroalgae [18]

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Conclusion