Climate change

The target of newspapers, political debates, and a rapidly growing branch of scientific research, climate change is often the forefront of discussion; both in, and out of academia. The consequences of human activity, or anthropogenic effects, on the ocean are multifaceted and have many potential consequences for the life it supports. Arguably, the two most significant changing conditions in the world’s oceans are temperature and pH, the effects of these in driving diversity will be explored in this article.

The culmination of research from multiple countries has resulted in a widely accepted view of the current and historical extent of ocean acidification and warming (see figure below). The Intergovernmental Panel on Climate Change (IPCC) have also developed projections for the continuation of these trends. There are four sets of projections, describing greenhouse gas emissions and their subsequent effects on the ocean, ranging from the  the “best case scenario” in terms of controlling greenhouse gas emissions, and the “current rate” formulated from projections of emissions and population growth continuing at its current rate. The graphic below from the IPCC shows that it is likely that the pH will continue to decrease, and temperature will continue to increase, making the oceans an increasingly challenging habitat to survive in.

Graph showing the a) projected increase in surface water temperature and b) decrease in surface water pH from 1950, projected until 2100, where the grey line is historical data, the red line is the “current rate scenario”and the blue is the “best case scenario”. Adapted from IPCC AR5 for policy makers report.

Considered a generally stable habitat, the ocean experiences overall little variation in physical conditions. The high specific heat capacity of water results in daily and annual temperature fluctuations a fraction of those seen in air. Resulting in marine organisms often being ill adapted to extremes in thermal range. In some regions, rising ocean temperatures are beginning to creep above the inhabitants metabolically sustainable body temperatures, towards lethal limits. This is particularly notable in the Antarctic, a region where annual variability of temperature is as low as 2ºC. Organisms which live there can be described as stenothermal; evolved to a precise, stable temperature range. It is these species with the smallest ranges in metabolically sustainable body temperatures which are most affected by climate change.

Weeds – Free from fragile connotations.

Tolerant species which are commonly pioneers or inhabit a wide range of habitats can be described as “weedy” or weed-like. Much like the weeds that grow out of cracks in pavement weedy organisms grow quickly and are tolerant of adverse conditions. These robust organisms commonly act as pioneer species, the first colonisers of disturbed habitats. The exploitative nature of these organisms often results in dominance after a disturbance event or when the existing populations are under stress.

Weeds in action:

Algae on coral reefs

Early stages of algal overgrowth on bleached coral, Great Barrier Reef. Justin Marshall:

Tropical corals rely, at least in part, on the production of organic compounds for nutrition by symbiotic microalgae (zooxanthellae) which live in their cells. Fluctuations in pH or temperature often leads to the expulsion of these microalgae resulting in the weakening, and often, ultimate death of the coral. This is known as coral bleaching. When corals bleach they are more vulnerable to large brown algae growing over them(see image right). These tropical corals are vulnerable to being smothered, and therefore there is a high associated mortality rate with algal growth over a coral. Pollution of fertiliser from agricultural run-off is a driver of eutrophication, this process effectively fertilises the water accelerating algal growth. The result of this is a regime shift of the community becoming algae dominated rather than coral dominated, reducing biodiversity.

Antarctic bryozoans

Due to the thermal stability of this ocean and regional accelerated rate of warming compared to the rest of the world’s waters, the Antarctic Ocean has become a site of warming experiments. One such experiment placed a 1ºC warming plate and control, un-warmed plate on the sea bed in the summer season, to observe the effect of temperature on the rate of colonisation. These experiments showed a loss of diversity, due to the dominance of one species of pioneer bryozoan (Fenestrulina rugula) – an encrusting, calcified, colonial body of small animals – which, without suffering negative effects of the warming, appeared to exploit the lack of competition form the other species. It is this “break-out” dominance of the hardier, “weedier” species in the ocean which is likely to cause the less robust species to be outcompeted for space and food.

What does it mean?

There is a trend between changing climatic conditions and diversity loss, as the less robust species are outcompeted by the “weeds” of the ocean. Biodiversity is the foundation supporting the familiar characteristics of the world’s marine habitats. Mankind relies heavily upon marine habitats: ecosystem services filter water, remove surplus carbon industrially produced from the atmosphere, and provide food. The connectivity of the ocean means habitats as picturesque as tropical coral reefs, or as remote as the plankton communities in the Antarctic, are fundamental to modern living and therefore biodiversity loss has potential to greatly impact the daily lives of many.

However, this lowered diversity is by no means permanent. There have been five mass extinctions in recorded natural history, yet, the earth’s biodiversity has always recovered. Loss of biodiversity in the oceans may open niches for the evolution of new species. Unfortunately, the time-line for this recovery ends outside of the foreseeable future so predictions on the matter cannot be investigated or observed, one thing is for certain; once a species is gone it won’t come back. Despite this outlook, it certainly isn’t a reason to ignore conservation, and carbon dioxide emission regulations. Rather, this visible downgrading of the biodiversity in marine habitats should be a motivating factor to reducing human impact on the planet.

Learn more

In this video explaining anthropogenic drivers of global biodiversity loss:

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