Metabolic theory

A sweeping study of marine phytoplankton has brought into question the idea that global ocean productivity is declining because of climate change.

Swiss scientists who claim to have completed “the first analysis of marine phytoplankton species richness and its ecological drivers at the global scale”  using more than 1 million observations of 1,300 species report that at this time the “metabolic theory of ecology” still applies.

At the ecosystem level, the metabolic theory links total resource biomass to temperature and decrees that production should go up as temperature increases.

“Consistent with metabolic theory, phytoplankton richness in the tropics is about three times that in higher latitudes, with temperature being the most important driver,” the Swiss scientists concluded in their peer-review study published in Science Advances days ago.

The study of the “Global pattern of phytoplankton diversity driven by temperature and environmental variability” was led by Damiano Righetti of the Swiss Federal Institute of Technology in Zurich with colleagues from the Swiss Federal Research Institute. 

Phytoplankton are the plants of the ocean. As such, they provide “the foundation of the aquatic food web, the primary producers, feeding everything from microscopic, animal-like zooplankton to multi-ton whales,” notes the NASA Earth Observatory. “Small fish and invertebrates also graze on the plant-like organisms, and then those smaller animals are eaten by bigger ones.”

A 2010 study in Nature concluded phytoplankton concentrations have been steadily declining since 1899 due to ocean warming, but the latest study found phytoplankton richness highest in the warmest part of the ocean.

“Analyzed by latitude, richness declines steeply poleward of 30 degrees, reaches its minimum ( approximately 50 species) and associated inflection points at mid-latitudes (between  45- to 65-degrees N and approximately 45 degrees S), and increases slightly toward the poles,” Righetti writes. “This latitudinal pattern is composed of species with notable wide thermal ranges (15.8° ± 6.8°C) and broad geographic distributions, with more than 60 percent of high-latitude species (those ranging as far as 70 degrees N and S) recorded close to the equator as well.”

The study might shed some light on the findings of Pacific Northwest scientists Greg Ruggerone and James Irvine, who last year reported North Pacific salmon populations are now at record highs, and help explain the phenomenal salmon runs Alaska witnessed during The Blob years when the surface waters in the Gulf of Alaska were unusually warm.

“Sea surface temperature (SST) is the most important driver for phytoplankton richness in our data,” Righetti observed. “It explains more than two-thirds of the global variation in diagnosed richness and is the most powerful predictor for species richness in the underlying raw observations.”

Warm water boost?

When SSTs were high during The Blob years, Alaska salmon harvests averaged close to 205 million fish per year – more than twice the 100 million salmon once considered the mark of a good season. 

The harvest, however, was largely comprised of pink salmon.

“Although the relationship between climate and pink salmon survival is likely complex, fluctuations in abundance appear to be modulated in large measure directly and indirectly by the thermal environment in which a stock lives,” University of Alaska Fairbanks scientist Alan Springer and colleague Gus van  Vliet from Auke Bay have theorized. “Such a fundamentally bottom-up explanation is bolstered by observations of high growth and survival rates of pink salmon during the period of warmer ocean temperatures and population increase.”

Chinook (king) and coho (silver) salmon, meanwhile, have trended downward as the ocean has warmed. Whether this is related to climate or ocean competition is unclear. Ruggerone has argued there is more evidence to support the latter than the former.

“…Pink salmon affected other species through exploitation of prey resources…,” he wrote in a paper for Reviews in Fish Biologist and Fisheries. “…Competition was observed in nearshore and offshore waters of the North Pacific Ocean and Bering Sea, and one study documented competition between species originating from different continents. Climate change had variable effects on competition. In the North Pacific Ocean, competition was observed before and after the ocean regime shift in 1977 that significantly altered abundances of many marine species, whereas a study in the Pacific Northwest reported a shift from predation- to competition-based mortality in response to the 1982/1983 El Nino.

“Key traits of pink salmon that influenced competition with other salmonids included great abundance, high consumption rates and rapid growth, degree of diet overlap or consumption of lower trophic level prey, and early migration timing into the ocean. The consistent pattern of findings from multiple regions of the ocean provides evidence that interspecific competition can significantly influence salmon population dynamics and that pink salmon may be the dominant competitor among salmon in marine waters.”

Definitive conclusions are, however, difficult because of the complexity of ocean ecosystems. The latest study on phytoplankton notes the confusing picture.

“Below 19 degrees C (66F), richness is lower than expected, with approximately 8 degree to 14 degree C (46 to 57F) waters (at approximately 35 degrees to 60 degrees latitude) showing the greatest divergence from theoretical predictions. Regions of reduced richness are characterized by maximal species turnover and environmental variability, suggesting that the latter reduces species richness directly, or through enhancing competitive exclusion. The nonmonotonic relationship between phytoplankton richness and temperature suggests unanticipated complexity in responses of marine biodiversity to ocean warming.”

The Gulf of Alaska sits between 55 degrees and 60 degrees North latitude. Gulf surface water temperatures have historically ranged from 3.9C (39F) to 13.4C (56F). It is an area influenced by a high degree of environmental variability – air and water temperatures being but two of the climate-related issues.

When it comes to phytoplankton, the Swiss study notes wind stress, deepwater mixing, nitrate levels, stratification, light penetration and more play roles. And the study underlines that there is a limit to temperature-fueled richness:

“…Richness responses to temperature may level off in the nutrient-poor tropical sea due to a slowdown of metabolic rates under nutrient scarcity, despite high temperatures,” it says.

The authors offer no assessment on how animal species within the ocean ecosystem might respond to changes in productivity at the plant level.

“Our study proposes a link of phytoplankton richness with both temperature and ocean variability; therefore, responses of global patterns in marine phytoplankton diversity to climate change may be more complex than hitherto anticipated, with possible impacts on higher trophic organisms, productivity, and ecosystem function,” the authors conclude.

What those impacts might be is unknown, and some higher trophic organisms – such as salmon – are best adapted to cooler waters. They would be expected to face trouble competing with warm water adapted species moving into the Gulf of Alaska if the ocean continues to warm significantly.

Ocean temperatures have increased by approximately 1.3 degrees C over the past 100 years, according to the International Union of Concerned Scientists (IUCN). And Alaska has already witnessed warm water species such as giant ocean sunfish, Pacific pomfret, market squid and Pacific bonito.