Fig 1.
BCM abundance along the coast of Curaçao and relationship with wave height and urbanisation.
(A) BCM abundance score (scale 1 to 5) averaged across the 4 surveys along the south coast of Curaçao. Populated and industrial areas are shown in dark blue and pink, respectively. Trade wind and water current are indicated by arrows. Stars locate the sites of low and high abundance of BCMs used as examples in the Results. The triangle locates the site of the in situ organic enrichment experiment. The diamond shows a site with GPS coordinates to locate the study area. (B) Contour plot showing the relationships between BCM abundance, wave height and urbanisation. Colours represent BCM score as shown in (A). Note that populated areas and BCM abundance are not obviously related in the large-scale map (A), especially in the West part of the island, but populated areas were often present along a narrow (ca. 500 m wide) strip of coast, which cannot be visualised in the map. This small-scale pattern was taken into account to score urbanisation levels used in the contour plot (B) (see Materials and Methods for details).
Fig 2.
Nutrient concentrations in the water column.
(A) Overview of different nutrient sampling locations at each site. (B) NOx and PO43- concentrations (mean ± SEM, n = 3–4 temporal replicates) as a function of season, ie. (W) warm/rainy (hatched) and (C) cold/dry (plain), depth, BCM abundance, ie. low (L) vs. high (H) abundance sites, and substrate type (as applicable) for surface, open ocean, intermediate and bottom waters.
Fig 3.
Organic matter content in sediments.
Percent organic carbon (Corg) (mean ± SEM) of (A) sediment cores collected far away (> 5 m) from any BCM patch at sites of low and high BCM abundance, and (B) sediment cores collected at Pestbay (site of high BCM abundance) in the middle, edge, next to (ca. 10cm away) and far away (> 5 m) from BCM patches. Analysed by ANOVA. Letters indicate homogeneous subgroups by posthoc Scheffe tests. Numbers of replicates are in parenthesis.
Fig 4.
Response of BCMs to organic matter enrichment.
(A) Representative photographs of the sediment surface for BCM seeded and non-seeded treatments with and without OM enrichment on days 0, 3, 7 and 12 (closed bucket). BCMs are visible as reddish brown coloration at the centre of the seeded buckets. (B) BCM coverage (mean ± SEM, n = 6) in open bottom and closed bottom buckets with initial BCM seed with and without OM enrichment over the duration of the experiment. Red arrows indicate when OM was added. Analysed by two-way ANOVA. Stars indicate significant differences.
Fig 5.
Oxygen concentrations in sediments across BCM patches.
(A) O2 concentration across BCM patches over an entire diel cycle. During photosynthesis, O2 peak concentrations per profile are plotted; otherwise surface concentrations are given (n = 350 profiles). (B and C) Examples of in situ O2 profile across the water-sediment interface during day and night with BCM (B) and without BCM (C).
Fig 6.
(A) Proposed model of sources and cycle of nutrients stimulating BCM growth. Nutrient inputs from land runoff (A1) or groundwater seepage (A2) cause benthic and planktonic phototrophic blooms. Fe is largely available as a result of long-term Fe addition by the African dust [54]. The blooms decay and produce particulate OM (B1) which settles on the seafloor as a function of wave action and current. The coral reef community takes up, produces and releases OM (B2) [18,33,50,68]. Reef degradation leads to a detritus-based food web and enhances OM accumulation on the seafloor. Increased OM loading leads to BCM growth via the release of nutrients from the microbial degradation of OM (C). (B) Schematic drawing of the water/sediment interface. Drawings from far away from (i), near to (ii), and middle of (iii) BCM patches are used as analogs for what we anticipate as OM accumulates over time. Increased OM concentration in the sediments results in a thinner oxygenated surface layer and an increased nutrient release, which triggers BCM growth. When BCMs develop, they produce OM and trap OM from the water column to sustain their growth and expand.