How does climate change affect the aquatic food chain?

Posted on September 23, 2013

 Although the effects of climate change on freshwater communities are  receiving increasing attention, we still have only a very sketchy idea of how climate warming might interact with other human impacts, such as nutrient pollution and the depletion of predator populations.  The independent and combined effects of these factors on the food web were compared in a Canadian experiment using 40 outdoor freshwater mesocosms, each 1.5 m in diameter and 0.6 m deep.  Each tank was inoculated with zooplankton, phytoplankton and microbes collected from nearby ponds.  Climate warming was simulated by using water heaters to raise the temperature in some tanks by 3oC, and nutrient enrichment was achieved by adding nitrogen and phosphorus  (264 and 27 μg/litre/month respectively).  A fish predator impact was created by adding threespine sticklebacks (Gasterosteus aculeatus; 5 / tank) to selected mesocosms.  To measure the effects of warming, nutrients and predators, the biomass of plant plankton was monitored weekly over a 16 month period.  Although the presence of fish generated a top-down food chain effect (trophic cascade), while the effect of nutrient fertilisation was bottom-up, both had strong positive effects on phytoplankton biomass.  In contrast, the impact of warming varied depending on the season of the year and the presence or absence of fish.  In winter and spring, warming of fish-less tanks encouraged a higher biomass of large-bodied grazing zooplankton, which depressed the biomass of algae.  And in summer, warming countered the positive effect of nutrients on algal biomass, possibly through its impacts on algal physiology and species composition.  Therefore,  taken together, the results suggested that higher temperatures tend to shift the control of primary production toward top-down control and away from bottom-up control.

 Reference:  Kratina, P. et al.  2012.  Warming modifies trophic cascades and eutrophication in experimental freshwater communities.  Ecology 93(6), 1421–1430.