Coral reef functioning in a highly variable environment : Effects of upwelling on Pacific coral reefs of Costa Rica

With increasing local and global stressors on coral reef ecosystems, the future stability of environmental conditions for coral reefs is in question. The capacity of reef organisms and ecosystems to adapt to such variable environments in terms of functions and services is a current topic in coral reef research, yet related knowledge, especially on the ecosystem level, is scarce. Marginal reefs that thrive in highly fluctuating or limiting environments, such as upwelling systems, provide natural laboratories to study the acclimatization and adaptation potential of reef organisms. This thesis aimed to contribute to the understanding of coral reef functioning in variable environments by investigating the impact of the Papagayo upwelling at the northern Pacific coast of Costa Rica. A series of interconnected studies was conducted in upwelling exposed reefs at weekly intervals over an entire observation year (April 2013 - April 2014). These studies monitored (i) a range of inorganic and organic water parameters, (ii) in situ primary production rates, (iii) benthic and herbivore community composition, and (iv) responses of algal and invertebrate settlement to simulated overfishing. Upwelling events between February and April 2014 decreased water temperatures by 7 - 9 °C for several hours or days, while inorganic nutrient concentrations increased 3 - 16-fold. Sequentially, organic matter concentrations in the water column above the reef doubled and remained elevated for 2 - 3 months (Chapter 2). Surprisingly, the strong seasonality in environmental conditions did not significantly influence benthic community productivity or composition in the studied reef. Upwelling-impacted water parameters negated each other in their effects on primary production of reef organisms. Corals were the only primary producers that benefitted from upwelling conditions, while production rates of all investigated algal taxa decreased (Chapter 3). Instead of following a seasonal cycle, benthic cover of the scleractinian corals Pocillopora spp. increased continuously and at an exceptionally high rate over the monitoring year (from 20 % to 50 % relative cover), while turf algal cover dropped significantly (from 60 % to 20 %). This shift in community composition was supported by high abundances of the herbivorous sea urchin Diadema mexicanum, which controlled turf algal cover in the reef community (Chapter 4). Fish exclusion significantly altered benthic communities on settlement tiles from short turf algae and crustose coralline algae dominance towards long turf algae, fleshy macroalgae and ascidians (Chapter 5). The results of this thesis indicate that the investigated benthic reef community is physiologically acclimatized to seasonal changes in environmental conditions, and that the local herbivore community effectively controls algae growth in the studied reef. However, high abundances of sea urchins potentially threaten the reef structure through bioerosion, and overexploitation of herbivorous reef fish may result in benthic community shifts. Additionally, repeated disturbances such as coral diseases (Chapter 6), El Niño events and harmful algal blooms likely prevent the coral community from increasing in diversity or developing a resistant reef structure. Upwelling-influenced reefs in the Gulf of Papagayo provide an example of how reefs may look like in the future, when anthropogenic chronic stressors will select for a minority of resilient coral species to dominate reef ecosystems. The results of this thesis suggest that those future reefs may still be highly productive and have a high resilience towards prevailing stressors in their ecosystem. Despite this, the reefs will be vulnerable to the intervention of acute stressors such as disease outbreaks or El Niño events due to low genetic diversity and functional redundancy in the coral populations