Abstract
Coral reefs are facing rapidly changing environments, but implications for reef ecosystem functioning and important services, such as productivity, are difficult to predict. Comparative investigations on coral reefs that are naturally exposed to differing environmental settings can provide essential information in this context. One prevalent phenomenon regularly introducing alterations in water chemistry into coral reefs are internal waves. This study therefore investigates the effect of large amplitude internal waves (LAIW) on primary productivity in coral reefs at the Similan Islands (Andaman Sea, Thailand). The LAIW-exposed west sides of the islands are subjected to sudden drops in water temperature accompanied by enhanced inorganic nutrient concentrations compared to the sheltered east. At the central island, Ko Miang, east and west reefs are only few hundred meters apart, but feature pronounced differences. On the west lower live coral cover (-38 %) coincides with higher turf algae cover (+64 %) and growth (+54 %) compared to the east side. Turf algae and the reef sand-associated microphytobenthos displayed similar chlorophyll a contents on both island sides, but under LAIW exposure, turf algae exhibited higher net photosynthesis (+23 %), whereas the microphytobenthos displayed reduced net and gross photosynthesis (-19 % and -26 %, respectively) accompanied by lower respiration (-42 %). In contrast, the predominant coral Porites lutea showed higher chlorophyll a tissues contents (+42 %) on the LAIW-exposed west in response to lower light availability and higher inorganic nutrient concentrations, but net photosynthesis was comparable for both sides. Turf algae were the major primary producers on the west side, whereas microphytobenthos dominated on the east. The overall primary production rate (comprising all main benthic primary producers) was similar on both island sides, which indicates high primary production variability under different environmental conditions.
Highlights
Coral reefs are highly diverse and productive ecosystems in a nutrient poor environment (e.g. 1,2,3)
These differences are likely caused by large amplitude internal waves (LAIW) that are distributed across the Andaman Sea in the direction of the Similan Islands
Despite the almost identical temperature mode and mean values measured at Ko Miang, daily temperature ranges (DTRs, daily maximum – minimum) were highest at W and in deep when compared to E and shallow (Table 1, analysis of variance (ANOVA) results Table S2 A)
Summary
Coral reefs are highly diverse and productive ecosystems in a nutrient poor environment (e.g. 1,2,3). The primary production of coral reefs is the basis of the food web, but so far only few attempts have been conducted to quantify primary production budgets in coral reefs (e.g. 8,9,10,11) Those studies give valuable insight on coral reef ecosystems in general, but may not provide information about primary production of today’s coral reefs subjected to a changing environment ([12,13,14]). The natural setting of the Similan Islands offers the unique opportunity to investigate primary production in benthic communities subjected to remarkably different environmental conditions only a few hundred meters apart. These differences are likely caused by large amplitude internal waves (LAIW) that are distributed across the Andaman Sea in the direction of the Similan Islands. Comparable to pulsed upwelling in other coastal areas ([23,24,25,26]), they may deliver essential nutrients to shallow water supporting primary production in an otherwise nutrient-depleted environment
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