Impact of Climate Change on Marine Biodiversity
Authors: Varun Saini^a, Ajay Kumar^b, Aman Jaiswal^b, Deepak Kumar Koli^b
Division of Entomology, ICAR-Indian Agricultural Research Institute, New Delhi (India) 110 012
Division of Microbiology, ICAR-Indian Agricultural Research Institute, New Delhi (India) 110 012

INTRODUCTION

Marine communities are biological networks in which the success of species is linked directly or indirectly through various biological interactions (e.g., predator-prey relationships, competition, facilitation and mutualism) to the performance of other species in the community. The aggregate effect of these interactions constitutes ecosystem function (e.g., nutrient cycling, primary and secondary productivity), through which ocean and coastal ecosystems provide the wealth of free natural benefits that society depends upon, such as fisheries and aquaculture production, water purification, shoreline protection, and recreation (Pinsky et al. 2013). However, growing human pressures, including climate change, are having profound and diverse consequences for marine ecosystems. The primary direct consequences are increasing ocean temperatures (Bindoff et al. 2007) and acidity (Doney et al. 2009). Climbing temperatures create a host of additional changes, such as rising sea level, increased ocean stratification, decreased sea-ice extent, and altered patterns of ocean circulation, precipitation, and freshwater input. In addition, both warming and altered ocean circulation act to reduce subsurface oxygen (O₂) concentrations (Keeling et al. 2010).

MAJOR PHYSICAL CHANGES IN OCEANS THROUGH CLIMATE CHANGE

Widespread changes in sea level, ocean pH and the extent of oxygen-deficient dead-zones are underway (Rahmstorf, 2007). In many instances these and other factors will impact together, creating negative synergistic effects to which organisms and ecosystems may have little resistance.

Ocean Acidification

One of the main impacts of ocean acidification on marine life arises because of interactions between acidity and carbonate availability. A taxonomically diverse array of marine organisms, including tiny coccolithophores (a type of phytoplankton), pelagic and benthic mollusks, fist-sized starfish and urchins, as well as massive corals, require calcium carbonate for their skeletons, and others have key carbonate rich structures (Hall-Spencer et al. 2008). All of these are likely to suffer as increasing acidity reduces carbonate availability, and impacts at the species level may cascade through to widespread community change.

Reducing dissolved oxygen concentration

Oxygen solubility in seawater is a function of temperature, and O ₂ availability in the world ocean has been declining since the 1950s as the ocean has warmed. Over a range from 0 to 15˚C, dissolved oxygen concentration in seawater is related approximately linearly to temperature, and will decline by about 6% per one degree rise (Vaquer-Sunyer and Duarte, 20080. Ongoing warming together with rising CO ₂ will see an expansion of low oxygen zones, perhaps by more than 50% of their present volume by the end of the century. Mobile organisms are able to avoid low oxygen concentrations, but sedentary organisms have little choice but to tolerate low oxygen concentrations or die.

Sea level rise

Global temperature influences water and ice volumes and hence, sea level. Sea level influences the inundation and establishment of coastal habitats and ecosystems (Peters, 2008). The rate of sea level rise during the 20 ^th century was proportional to the warming above pre-industrial temperatures, and extrapolation suggests further rises of between 0.5 and 1.4 m above 1990 levels by 2100. Sea level changes impact habitat space, drive speciation, influence biodiversity and alter local nutrient flux (Rahmstorf, 2007).

IMPACTS OF CLIMATE CHANGE ON MARINE ECOSYSTEMS:
SELECTED CASE STUDIES

Ice-dominated polar systems

Life cycles and physiological requirements of many polar organisms are closely tied to the annual cycles of sea ice and available sunlight. Model projections reveal that greater light availability caused by a reduction in sea ice may increase open-water phytoplankton primary production (Steinacher et al. 2010), although nutrient limitation could ultimately limit the magnitude of this increase. Sea ice also provides an important habitat for seabirds and mammals (e.g., polar bears, walrus and seals) that use the ice as a foraging platform or breeding habitat, suggesting that these species will face problems with warming. Arctic ecosystem responses to climate change (Wassmann et al. 2011) documented marine species range shifts; changes in abundance, growth, condition, behavior, and phenology; and community and regime shifts as key components of change. Thus, climate change can drive changes in both abiotic and biotic interactions, including nonlinear impacts within the food-web structure that can have unexpected results that may move current marine ecosystems to a new state (Grebmeier et al. 2010).

Open ocean environments

In the North Atlantic, phytoplankton biomass might collapse by half and, because most zooplankton populations there are subject to bottom-up control (limited by availability of their phytoplankton food), secondary production would be much reduced. Fisheries production would also be expected to decline (Cheung et al. 2008). Increasing CO₂ and temperature may increase phytoplankton growth rates, but, at the same time, may lead to physiological responses that render cells more susceptible to UV damage. Ongoing change might be expected to impact primary production, particularly in coastal seas where river runoff supplements nutrient availability and where sea level rise might also impact nutrient loading.

In addition to fueling food chains, phytoplankton growth draws CO ₂ down from the atmosphere, driving the biological pump that transports carbon to the ocean interior. This flux will change as temperatures increase because nutrient reduction leads to smaller sized cells that sink more slowly (Winder et al. 2008).

Coral reef systems

Coral reefs are among the most diverse and economically important ecosystems on Earth. They are threatened by a variety of climate-related changes, including rising sea temperatures and levels as well as acidification. Corals require calcium carbonate (in the form of aragonite) to build skeletons, but acidification is driving availability down. Calcification of Great Barrier Reef corals has declined by 14.2% since 1990 (Death et al. 2009). At the present rate of CO2 increase, carbonate accretion is expected to be further compromised such that corals will become increasingly rare beyond 2050. Corals and, in particular, their symbiotic zooxanthellae algae are highly sensitive to increases in temperature. Above 31˚C, zooxanthellae are ejected and coral bleaching ensues. The intensity and scale of bleaching has increased since the 1960s, and major bleaching events in 1998 and 2002 affected entire reef systems.

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About Author / Additional Info:
I am pursuing MSc in department of Entomology from Indian Agricultural Research Institute, New Delhi-110012