Coral reefs contain a diverse consortium of benthic macro-organisms including scleractinian corals, fleshy algae, calcifying algae, and their associated microbiotas. These organisms are in a constant battle, competing for space and resources. Over the last 50 years, fleshy algae have increasingly been shown to outcompete corals; however, the mechanisms are not completely known. Algal-derived dissolved organic matter (DOM) can induce mortality of reef building corals. One proposed killing mechanism is a zone of hypoxia created by rapidly growing microbes. To investigate this hypothesis, oxygen optodes were used to investigate dissolved oxygen concentrations at coral?algal interfaces, as well as how the biological oxygen demand (BOD) of coral-associated microbial communities are affected by algal DOM. Oxygen optodes were visualized with a novel, low-cost Submersible Oxygen Optode Recorder (SOOpR) system, which is capable of accurately measuring oxygen concentrations in the lab or in situ. The BOD studies examined the effects of two types of algal organic matter; turf algae, shown to be detrimental to corals, and crustose coralline algae (CCA), which have been shown to be beneficial. This investigation of oxygen dynamics of coral-algal interactions shows that algae can create complex oxygen dynamics that change with flow, and when in close contact with coral, the interface becomes hypoxic. When microbial communities from coral are exposed to turf algal exudate and CCA exudate, exudates from turf algae elicit the greatest BOD. Together, the results of these studies show that coral-algal interaction zones can be hypoxic, and this is due in some part to microbial activity.