Abstract
Coral reef degradation increases coral interactions with filamentous turf algae (FTA) and macroalgae, which may result in chronic stress for the corals. We evaluated the effects of short (2.5 month) and long (10 month) periods of FTA removal on tissue thickness (TT), zooxanthellae density (ZD), mitotic index (MI), and concentration of chlorophyll a (Chl a) in Montastraea annularis at the beginning and end of gametogenesis. Ramets (individual lobes within a colony) consistently surrounded by FTA and ramets surrounded by crustose coralline algae (CCA) were used as controls. FTA removal reduced coral stress, indicated by increased TT and ZD and lower MI. The measured effects were similar in magnitude for the short and long periods of algal removal. Ramets were more stressed at the end of gametogenesis compared with the beginning, with lower ZD and Chl a cm−2, and higher MI. However, it was not possible to distinguish the stress caused by the presence of FTA from that caused by seasonal changes in seawater temperature. Ramets surrounded by CCA showed less stress in comparison with ramets surrounded by FTA: with higher TT, Chl a cm−2 and ZD, and lower MI values. Coral responses indicated that ramets with FTA suffered the most deleterious effects and contrasted with those measured in ramets surrounded by CCA. According to published studies and our observations, there could be at least six mechanisms associated to FTA in the stress caused to M. annularis by FTA. Owing to the high cover of FTA (in contrast to macroalgae and CCA) in the Caribbean, the chronic stress, the overgrowth and mortality that this functional algal group can cause on M. annularis species complex, a further decline of this important reef-building coral in the Caribbean is expected.
Highlights
Coral reef degradation can involve a dominance shift from live coral colonies to dead corals overgrown by algae [1,2]
Ramets permanently surrounded by filamentous turf algae (FTA) (T3) showed the lowest mean values of tissue thickness (TT) and zooxanthellae density (ZD), and high mitotic index (MI) compared to to months (T1) and T4 (Table 1, Figure 3)
The ramets surrounded by crustose coralline algae (CCA) and those with FTA removal for long (T1) and/or short (T2) periods had similar values of ZD and MI
Summary
Coral reef degradation can involve a dominance shift from live coral colonies to dead corals overgrown by algae [1,2]. An increase in algal cover on degraded coral reefs has led to a high frequency and longer duration of interactions between corals and algae [2,6,7]. Symptoms of coral stress caused by the presence of those algae include hypoxia, tissue disruption, altered pigmentation [13,16,17], bleaching [13], increment in thickness of the diffusive boundary layer [18], a major shift in the bacterial communities in the algae-coral tissue interaction (including pathogenic microbes) [17], reductions in fecundity, photosynthetic performance and growth rate [13,19,20,21,22,23], and mortality [16,24]. Ramets of Montastraea annularis bordered by CCA showed no deleterious responses Instead, they had a slightly higher photochemical efficiency, higher concentrations of dissolved oxygen (hyperoxia), and an increase in the diversity of coral-associated bacterial communities, including non-pathogenic microbes [17,23]
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