Impacts of climate change on aquatic ecosystems
disruption of marine food webs
The slowing down of ocean circulation can disrupt marine food webs. Ocean circulation transports heat from the tropics to the poles and allows for the exchange of nutrients and oxygen between the surface and deep waters. However, as the sinking of water is reduced at the poles, warm surface waters mix less with cooler, deeper water.
With reduced mixing of deep and surface waters, aquatic organisms living near surface waters, especially phytoplanktons, would not get their supply of nutrients from the deep ocean. As phytoplanktons form the base of the food web, the species higher up in the food chain, such as fish and whales, will decrease due to a lack of food. The entire ecosystem will be affected.
It has been found that phytoplankton levels in the North Atlantic have dropped by 10% since the 1800s.
Similarly, the increase in ocean temperatures can have huge consequences on aquatic ecosystems. The warming oceans are expected to result in rapid changes in aquatic environments. Some species are not able to adapt and are therefore at risk of going extinct. As species are dependent on one another for food, a decline in one or more species may cause entire ecosystems to collapse. The increase in ocean temperatures will result in the following changes that can in turn disrupt marine food webs:
i. Changes in geographic distribution of aquatic species
In response to climate change, aquatic species migrate to areas where temperatures are more favourable. In general, this involves aquatic species moving polewards and into deeper, cooler waters due to rising ocean temperatures. For example, due to rising ocean temperatures, aquatic species such as the the Atlantic Cod are moving polewards and into deeper, cooler waters.
Hence, it is expected that there will be a decrease in biodiversity in the equatorial regions, with an increase in biodiversity in poleward regions. A study found that the diversity of species in tropical waters such as fish has dropped significantly in the last 50 years.
However, this poleward movement is a general trend. There are some exceptions to this general trend. As the oceans do not warm uniformly, cool spots of water can be found in the lower latitudes. Therefore some species have been found to migrate away from the poles into these cooler spots of water.
For example, in the US, some fish species such as the Witch flounder have shifted south, towards the lower latitudes, to these cool spots of water.
ii. Changes in population density of species and composition of ecosystems
As the geographic distribution of species changes, the mix and population densities of prey, predators and competitors in the ecosystems also change. This disrupts existing food webs, and new aquatic communities develop. As aquatic ecosystems are complex, exactly how they might evolve is still uncertain, though some changes have already been observed. Scientists believe that some species would flourish and others would not. Some species might find an abundance of suitable sources of food, resulting in an increase in their population. On the other hand, the species that they feed on might decline.
In addition, species that are unable to migrate or move fast enough may not be able to adapt to the higher sea temperatures. They may face local or global extinction as their environment becomes unsuitable for them, and their habitats and food sources disappear.