The ocean, a vast and dynamic system, experiences periods of unusual and intense conditions known as marine extremes. These events represent significant departures from typical ocean states, often with profound implications for marine life and ecosystems. Scientists are increasingly focused on understanding these phenomena as their occurrence appears to be changing.
Understanding Marine Extremes
A marine extreme is defined as a prolonged, discrete period of unusually warm or cold water, or other anomalous conditions, at a particular location for that time of year. Unlike regular ocean variability, which includes daily or seasonal fluctuations, these events are characterized by their intensity, duration, and spatial extent. For instance, a marine heatwave is identified when ocean temperatures exceed the 90th percentile of historical measurements for at least five consecutive days in a given location and time of year.
Marine extremes push ecosystems beyond their normal coping ranges, leading to widespread impacts. Their intensity refers to how much temperatures deviate from the average, such as the 2014-2016 “Blob” in the North Pacific where temperatures were up to 6°C (11°F) higher than average. The duration of these events can range from days to months, and their spatial extent can cover hundreds to thousands of square kilometers.
Major Types of Marine Extremes
Ocean heatwaves are prolonged periods of unusually warm water. These events can be caused by various factors, including local atmospheric conditions, large-scale climate patterns, and global warming. For example, weakened winds that normally cool the ocean through evaporation can lead to the upper layer of the ocean heating more easily.
Ocean deoxygenation, a significant reduction in dissolved oxygen levels, occurs when warmer waters hold less oxygen or due to increased biological activity that consumes oxygen. Ocean acidification, characterized by a decrease in ocean pH, results from the absorption of excess carbon dioxide from the atmosphere into the ocean. This chemical change affects the availability of carbonate ions, which are building blocks for many marine organisms.
Other marine extremes include extreme cold snaps, where water temperatures drop significantly below seasonal averages, or freshwater pulses, involving sudden influxes of large amounts of fresh water into marine environments. These events, while perhaps more localized, disrupt ecosystems accustomed to specific salinity or temperature ranges.
Consequences for Marine Ecosystems
Marine extremes inflict widespread consequences across ocean ecosystems. They lead to significant shifts in marine biodiversity, causing species to relocate or resulting in localized extinctions for those unable to move. For example, non-mobile organisms like corals, algae, and sponges are particularly vulnerable to extreme thermal stress, often leading to mass mortality events.
These events disrupt food webs, impacting organisms from primary producers to top predators. Marine heatwaves, for instance, have been linked to harmful algal blooms and reduced chlorophyll-a concentration. Habitat integrity also suffers, as seen in widespread coral bleaching events, die-offs of seagrass beds, and the collapse of kelp forests, such as the decimation of kelp forests off western Australia in 2011. This habitat degradation diminishes the structural complexity and productivity of marine environments.
The impacts extend to ecosystem services. Fisheries experience declines in catches due to species migration or mortality, impacting local economies. Coastal protection provided by natural habitats like coral reefs and kelp forests is also compromised, increasing vulnerability to storms and erosion.
Survival Strategies of Marine Life
Marine organisms employ various strategies to cope with extreme conditions. Some species exhibit physiological adaptations, such as altering metabolic rates to conserve energy during stress, or producing stress proteins protecting their cells from damage. These internal adjustments allow them to tolerate conditions outside their usual range for a limited time.
Behavioral responses are common, including migration to areas with more favorable temperatures or seeking refuge in deeper, cooler waters or shaded microhabitats. For instance, some fish species may migrate to more temperate waters during prolonged marine heatwaves. Genetic resilience plays a role over longer timescales, as populations with greater genetic diversity can adapt through natural selection across generations.
Despite these coping mechanisms, the vulnerability and sensitivity of different species vary widely. Organisms with limited mobility, slow reproductive rates, or highly specialized habitat requirements are often more susceptible to the effects of marine extremes. The rapid increase in the frequency and intensity of these events overwhelm the adaptive capacities of many marine species, leading to population declines or ecosystem shifts.