Background
The authors write that "competition between corals and benthic algae is prevalent on coral reefs worldwide," and they say that "human activities may influence the outcome of these interactions by favoring algae to become the superior competitor," as "the ability of corals to fight off their algal competitors becomes increasingly important in the face of local stressors," such as fishing, eutrophication and sedimentation.

What was done
To further explore this subject, Barott et al. "surveyed the abundance, composition, and apparent outcome of different types of coral-algae competitive interactions on coral reefs in the Line Islands of the Central Pacific," where their survey sites "included reefs surrounding two nearly pristine uninhabited islands, as well as reefs experiencing a gradient of human activity, inorganic and organic nutrient regimes and microbial communities," as described by Dinsdale et al. (2008) and Sandin et al. (2008).

What was learned
The seven scientists determined that corals were "superior competitors against turf algae on reefs surrounding uninhabited islands," but that on reefs surrounding inhabited islands, "turf algae were generally the superior competitors." In explaining these findings, they suggest that "corals experiencing local human influences may be weaker competitors due to an increased abundance of potentially pathogenic bacteria and a higher prevalence of certain coral diseases," citing Dinsdale et al. (2008). In addition, they say that "algae may become better competitors on reefs surrounding inhabited islands due to increased inorganic nutrient concentrations that may increase fleshy algal growth or increase the abundance of pathogenic bacteria." At the other extreme, on the other hand, they say that "on reefs surrounding inhabited islands, increases in the success of turf algae over corals may be a result of a shift in reef fish community structure due to fishing pressure," citing DeMartini et al. (2008), Sandin et al. (2008) and Ruttenberg et al. (2011).

What it means
Barrott et al. conclude that their data suggest that "human disruption of the reef ecosystem may lead to a building pattern of competitive disadvantage for corals against encroaching algae, particularly turf algae, potentially initiating a transition towards algal dominance." And we would add that this extra pressure on earth's corals also makes them more susceptible to high-temperature-driven coral bleaching and the potential negative effects of ocean acidification.

References
DeMartini, E.E., Friedlander, A.M., Sandin, S.A. and Sala, E. 2008. Differences in fish-assemblage structure between fished and unfished atolls in the northern Line Islands, central Pacific. Marine Ecology Progress Series 365: 199-215.

Dinsdale, E.A., Pantos, O., Smriga, S., Edwards, R.A., Angly, F., Wegley, L., Hatay, M., Hall, D., Brown, E., Haynes, M., Krause, L., Sala, E., Stuart A. Sandin, S.A., Thurber, R.V., Willis, B.L., Azam, F., Nancy Knowlton, N. and Rohwer, F. 2008. Microbial ecology of four coral atolls in the Northern Line Islands. PLoS ONE 3: e1584.

Ruttenberg, B.I., Hamilton, S.L., Walsh, S.M., Donovan, M.K., Friedlander, A., DeMartini, E., Sala, E. and Sandin, S.A. 2011. Predator-induced demographic shifts in coral reef fish assemblages. PLoS ONE 6: e21062.

Sandin, S.A., Smith, J.E., DeMartini, E.E., Dinsdale, E.A., Donner, S.D., Friedlander, A.M., Konotchick, T., Malay, M., Maragos, J.E., Obura, D., Pantos, O., Paulay, G., Richie, M., Rohwer, F., Schroeder, R.E., Walsh, S., Jackson, J.B.C., Knowlton, N. and Sala, E. 2008. Baselines and degradation of coral reefs in the northern Line Islands. PLoS ONE 3: e1548.