Abstract
BackgroundTropical coral reefs cover ca. 0.1% of the Earth’s surface but host an outstanding biodiversity and provide important ecosystem services to millions of people living nearby. They are currently threatened by local stressors (e.g. nutrient enrichment and chemical pollution arising from poor land management, sewage effluents, agriculture, industry) and global stressors (mainly seawater warming and acidification, i.e. climate change). Global and local stressors interact in different ways, but the presence of one stressor often reduces the tolerance to additional stress. While global stressors cannot be mitigated solely by local actions, local stressors can be reduced through ecosystem management, therefore minimizing the impact of climate change on coral reefs. We systematically mapped the evidence of impacts of chemicals arising from anthropogenic activities on tropical reef-building corals, which are the main engineer species of reef ecosystems, to inform decision-makers on the available evidence on this topic.MethodsWe searched the relevant literature using English terms combined in a tested search string in two publication databases (Scopus and Web Of Science Core Collection). The search string combined terms describing the population (tropical reef-building corals) and the exposure (chemicals). We searched for additional literature through three search engines, three dissertations repositories, 11 specialist websites, and through a call to local stakeholders. Titles, abstracts, and full-texts were successively screened using pre-defined eligibility criteria. A database of all studies included in the map with coded metadata was produced. The evidence was described and knowledge clusters and gaps were identified through the distribution and frequency of studies into types of exposure and/or types of outcomes and/or types of study.Review findingsThe initial searches identified 23,403 articles which resulted in 15,177 articles after duplicate removal. Among them, 908 articles were retained after screening process, corresponding to 7937 studies (a study being the combination of a taxon, an exposure, and an outcome). Among these studies, 30.5% dealt with the impact of nutrient enrichment on corals while 25% concerned the impact of human activities without reference to a chemical. The most measured outcomes were those related to the chemical concentration in corals (bioaccumulation, 25.8%), to coral physiology (16.9%), cover (14%), and mortality (9%). Half of the studies (48.4%) were experimental—the exposure was controlled by the researchers—and were conducted in laboratory conditions (39.4%) and in situ (9%). The most studied taxa, exposure, and outcomes were different between experimental and observational studies.ConclusionsWe identified four well-represented subtopics that may be amenable to relevant full syntheses via systematic reviews: (1) evidence on bioaccumulation of chemicals by corals; (2) evidence on the effects of nutrient enrichment on corals; (3) evidence on the effects of human activities on corals; and (4) evidence on the ecotoxicological effects of chemicals on corals (except nutrient enrichment). The systematic map shows that corals in their natural environment can be exposed to many categories of chemicals, and that there is a complete gap in experimental research on the combined effects of more than two categories of chemicals. We therefore encourage research on this topic.
Highlights
Tropical coral reefs cover ca. 0.1% of the Earth’s surface but host an outstanding biodiversity and pro‐ vide important ecosystem services to millions of people living nearby
We identified four well-represented subtopics that may be amenable to relevant full syntheses via systematic reviews: (1) evidence on bioaccumulation of chemicals by corals; (2) evidence on the effects of nutrient enrichment on corals; (3) evidence on the effects of human activities on corals; and (4) evidence on the ecotoxicologi‐ cal effects of chemicals on corals
The systematic map shows that corals in their natural environment can be exposed to many categories of chemicals, and that there is a complete gap in experimental research on the combined effects of more than two categories of chemicals
Summary
The systematic map followed the Collaboration for Environmental Evidence Guidelines and Standards for Evidence Synthesis in Environmental Management [30] and the protocol has been published in Environmental Evidence [31]. Data coding strategy All articles included in the map were split into studies, i.e. the combination of a taxon, an exposure, and an outcome, and the following information was recorded in Microsoft Excel sheet from full-texts (details are given in Additional file 6):. The outcomes most often studied were coral cover (48.2%), mortality (10.2%), and disease (10.1%, Fig. 7) Because these studies provided no information on the chemicals that may explain the observed effects on corals, they should be synthetized separately, and can be the focus of a systematic review indicating how various human activities (e.g. urbanisation, tourism, agriculture, industries) impact corals. Limitations of the map Limitations of the synthesis method Firstly, we found a high number of syntheses/reviews/ meta-analyses that met our eligibility criteria (285 articles listed in Additional file 5) They may contain a substantial number of references that were not retrieved by our literature searches. The Philippines is ranked third in the world in terms of reef area (9% of the world’s total coral reef area, [29])
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