Biogeochemical response of tropical coastal systems to present and past environmental change

Global climate and environmental change affect the biogeochemistry and ecology of aquatic systems mostly due to a combination of natural and anthropogenic factors. The latter became more and more important during the past few thousand years and particularly during the ‘Anthropocene’. However, although they are considered important in this respect as yet much less is known from tropical than from high latitude coasts. Tropical coasts receive the majority of river inputs into the ocean, they harbor a variety of diverse ecosystems and a majority of the population lives there and economically depends on their natural resources. This review delineates the biogeochemical response of coastal systems to environmental change and the interplay of natural and anthropogenic control factors nowadays and in the recent geological past with an emphasis on tropical regions. Weathering rates are higher in low than in high latitude regions with a maximum in the SE Asia/Western Pacific region. On a global scale the net effect of increasing erosion due to deforestation and sediment retention behind dams is a reduced sediment input into the oceans during the Anthropocene. However, an increase was observed in the SE Asia/Western Pacific region. Nitrogen and phosphorus inputs into the ocean have trebled between the 1970s and 1990s due to human activities. As a consequence of increased nutrient inputs and a change in the nutrient mix excessive algal blooms and changes in the phytoplankton community composition towards non-biomineralizing species have been observed in many regions. This has implications for foodwebs and biogeochemical cycles of coastal seas including the release of greenhouse gases. Examples from tropical coasts with high population density and extensive agriculture, however, display deviations from temperate and subtropical regions in this respect. According to instrumental records and observations the present-day biogeochemical and ecological response to environmental change appears to be on the order of decades. A sediment record from the Brazilian continental margin spanning the past 85,000 years, however, depicts that the ecosystem response to changes in climate and hydrology can be on the order of 1,000-2,000 years. The coastal ocean carbon cycle is very sensitive to Anthropocene changes in land-derived carbon and nutrient fluxes and increasing atmospheric carbon dioxide. As opposing trends in high latitude regions tropical coastal seas display increasing organic matter inputs and reduced calcification rates which have important implications for calcifying organisms and the carbon source or sink function of the coastal ocean. Particularly coral reefs which are thriving in warm tropical waters are suffering from ocean acidification. Nevertheless, they are not affected uniformly and the sensitivity to ocean acidification may vary largely among coral reefs. Therefore, the prediction of future scenarios requires an improved understanding of present and past responses to environmental change with particular emphasis put on tropical regions.

Energetic plasticity underlies a variable response to ocean acidification in the pteropod, Limacina helicina antarctica

Ocean acidification, caused by elevated seawater carbon dioxide levels, may have a deleterious impact on energetic processes in animals. Here we show that high PCO₂ can suppress metabolism, measured as oxygen consumption, in the pteropod, L. helicina forma antarctica, by ~20%. The rates measured at 180–380 µatm (MO₂ = 1.25 M^−0.25, p = 0.007) were significantly higher (ANCOVA, p = 0.004) than those measured at elevated target CO₂ levels in 2007 (789–1000 µatm, = 0.78 M^−0.32, p = 0.0008;). However, we further demonstrate metabolic plasticity in response to regional phytoplankton concentration and that the response to CO₂ is dependent on the baseline level of metabolism. We hypothesize that reduced regional Chl a levels in 2008 suppressed metabolism and masked the effect of ocean acidification. This effect of food limitation was not, we postulate, merely a result of gut clearance and specific dynamic action, but rather represents a sustained metabolic response to regional conditions. Thus, pteropod populations may be compromised by climate change, both directly via CO₂-induced metabolic suppression, and indirectly via quantitative and qualitative changes to the phytoplankton community. Without the context provided by long-term observations (four seasons) and a multi-faceted laboratory analysis of the parameters affecting energetics, the complex response of polar pteropods to ocean acidification may be masked or misinterpreted.

L’acidification des océans nuit à la production des huîtres (in French)

Une étude menée dans le Pacifique établit pour la première fois en milieu naturel le lien entre acidification et mortalité des naissains.

On connaissait les expériences menées en laboratoire, mais c’est la première fois que des chercheurs réussissent à montrer en milieu naturel l’effet néfaste de l’acidification des océans sur les huîtres. C’est ce que révèle une étude publiée dans la revue Limnology and Oceanography.

L’alerte remonte à 2007. Dans une des grandes écloseries d’huîtres de l’Oregon (États-Unis), fournissant les ostréiculteurs de la côte du Pacifique, les naissains ont commencé à dépérir. Les millions de bébés huîtres Crassostrea gigas, une fois plongés dans la mer, mouraient les uns après les autres. Une perte économique se comptant en millions de dollars.