Shedding light on DOC release by benthic primary producers and its consumption by bioeroding sponges

Abstract

The effect of light on dissolved organic carbon (DOC) release of benthic primary producers (BPPs) was investigated on the coral reefs of Curaçao. Incubation experiments revealed a positive relation between the DOC release of four Caribbean reef algae (Cladophora sp., Dictyota menstrualis, Lobophora variegata, turf algae) and light availability (light-driven DOC release). In contrast, the DOC release of two tested scleractinian coral species (Madracis mirabilis, Orbicella annularis) was either minimal or DOC uptake occurred independent of light availability. Complementary in situ measurements in close proximity to BPPs and the water column showed a positive correlation between DOC concentrations and light intensity. Furthermore, average DOC concentrations were twice as high during the sampling in May-June compared to the sampling in November-December. This can at least be partly explained by a significantly higher light availability in May-June. However, in situ DOC concentrations near the reef alga Dictyota sp. did not follow a natural light gradient from 5 to 20 m depth, suggesting that apart from light availability also other factors such as photoinhibition and water movement may affect DOC concentrations near BPPs. Additional incubations were performed with turf algae under elevated nutrient conditions. Interestingly, when additional nutrients were added, DOC release at low and high light intensities were similar (light-independent DOC release). The availability of nutrients therefore determined whether light-driven or light-independent DOC release occurred. Similar to reef algae benthic cyanobacterial mats (BCMs) displayed high DOC release rates in in situ incubations during the day. However, DOC release at night was more than twice that high. This DOC release in the dark is proposed to be the result of incomplete organic matter degradation and fermentation under anoxic conditions. When integrating the day and night DOC release over a 24 hrs diurnal cycle, BCMs (benthic cover: 24%) were estimated to release four times more DOC than reef algae (benthic cover: 36%). BCMs thereby display the highest contribution to the benthic DOC production (80%). Excavating sponges are the most important bioeroders on Caribbean reefs. If they, similar to other sponges, can directly feed on DOC, they may benefit from an increase in DOC production due to ongoing coral-algal phase shifts. Therefore, the potential DOC and bacterial uptake of the common excavating sponges Siphonodictyon sp. and Cliona delitrix were investigated in situ. Despite high bacterial retention efficiencies, both species relied mainly on the uptake of DOC to meet their carbon demand. This utilization of DOC indicates that excavating sponges might benefit from an increase in DOC production as result of coral-algal phase shifts.