In their paper, de Senerpont Domis et al. (2013) discussed possible changes in plankton dynamics in temperate, polar and tropical regions resulting from climate change. According to the authors, this paper is an extension of the model designed by the Plankton Ecology Group (Sommer et al., 1986), initially developed in temperate regions, to other latitudes, polar and tropical.
Most of the argumentation regarding the effect of climate change is based on changes in temperature and precipitation. Changes in temperature are not questionable, as the IPCC projections point towards a general temperature increase in all latitudes. However, the forecast for precipitation in tropical regions is not as straightforward as it may appear from this paper due to the fact that local or regional features might be stronger constraints than global climate to biological activity. Regional projections from the IPCC (Meehl et al., 2007) for tropical regions (between 23ºN and 23ºS) did not forecast major changes in precipitation, as it might seem in fig. 2 in the paper by de Senerpont Domis et al.
The expected changes in precipitation in tropical regions are practically insignificant compared with higher latitudes and restricted to some specific areas. In the tropics, the only regions where precipitation is projected to increase are eastern Africa, South-East Asia (Table 1) and possibly the extreme north-west of South America, in the coast of Peru and Ecuador (Marengo et al., 2010). Regional climate models for South America predict that rather than higher precipitation, most areas will experience higher frequency of extreme rainfall (storm) events but also longer periods of drought (Marengo et al., 2010). These changes would decrease even more the predictability of seasonal rainfall and associated processes, when relevant (e.g. floodplain cosystems), and thus, the ecolog-ical consequences should be studied in shorter-scale approaches (Roland et al., 2012).
|Months||Precipitation Response (%)|
12S,20W to 22N,18E
12S,22E to 18N,52E
11S,95E to 20N,115E
10N,116W to 30N,83W
20S,82W to 12N,34W
The generalised seasonal development of current and future phytoplankton and zooplankton biomass presented in fig. 3 in the paper by de Senerpont Domis et al. does not reflect a general pattern for tropical lakes, but may only be valid for lowland, shallow, floodplain lakes usually observed in large tropical river basins. None of the 21 models of the IPCC predict an increase in precipitation in the Amazon and very little in the Congo basin, where these lowland floodplain lakes are most common. Thus, the prediction of phytoplankton dynamics changes as a result of higher rainfall may be incorrect.
In eastern Africa, which is the only region where precipitation would actually increase according to the IPCC (Table 1), most lakes are large and deep. Given their size, long retention time and small catchments, these large lakes will not be affected by flushing, as suggested by de Senerpont Domis et al. (2013). On the contrary, it has been well accepted for decades that internal loading is the major process affecting nutrient availability in large tropical lakes (Kilham & Kilham, 1990). The major effect of global and regional warming on these lakes has been an increase in temperature-driven density gradients due to warmer surface temperature with a subsequent increase in water column stability. At the same time, reduced wind speed has decreased vertical mixing and nutrient fluxes from internal loading (affecting mainly P inputs), resulting in reduced primary production. This has been evidenced in several studies on Lake Tanganyika, East Africa, based on recent data (Verburg, Hecky & Kling, 2003, 2006; Stenuite et al., 2007) as well as palaeolimnological records (e.g. O'Reilly et al., 2003; O'Reilly, Dettman & Cohen, 2005; Cohen et al., 2006; Tierney et al., 2010). Thus, despite the slight increase in precipitation that might occur, the effect of climate change alone on eastern African great lakes is oligotrophication, rather than an increase in planktonic productivity. However, this reasoning holds for large, deep lakes with a permanent hypolimnion, whereas shallower lakes may respond to climate change in a different way. It is also obvious that other anthropogenic impacts may override the effects of climate change and totally invalidate predictions based solely on climate. For instance, eutrophication of Lake Victoria resulted from multiple stresses including population growth, increased land use, exotic species introduction and meteorological variability (Hecky et al., 2010).
In South America, there are three distinct climate change scenarios, with respect to precipitation patterns, that could affect ecosystem functioning (Roland et al., 2012): zone 1, western Amazon and sub-tropical region with a slight increase in precipitation; zone 2, east Amazon and north-east (semi-arid region) with lower precipitation; and zone 3, south-east and coastline with similar precipitation amounts but increased frequency of storms. The aquatic ecosystems present in these three areas differ in several ways (e.g. with regard to frequency of connection of river to the ocean, to their humic content, and food-web structure), so that the interactions of the predicted changes with climate would be certainly different. The only zone that might follow the changes predicted by de Senerpont Domis et al. is zone 1 (i.e. western Amazon) which is dominated by rivers and oxbow lakes. Thus, lake type and regional characteristics should be taken into account for more precise predictions of changes in ecosystem function as a consequence of climate change.
Overall, the discussion throughout the paper concerning the tropical regions might be biased. The authors did not take into account the specificity of tropical systems, and some of the studies extensively cited by de Senerpont Domis et al. (e.g. studies comparing subtropical versus temperate systems, or long-term warming experiments in mesocosms conducted in northern Europe) are not the most relevant to describe and even less to predict plankton dynamics in tropical regions. We believe that more studies in tropical lakes, both large and small, in different regions, are still necessary to infer the impact of climate change on these ecosystems.