Jun 14, 2013
Grazing zooplankton could be key to marine chlorophyll decline
A much discussed 2010 study reported that chlorophyll concentrations in the world's oceans – an indicator of phytoplankton growth – declined over the 20th century at a rate of around 1% a year. Physical factors associated with climate change, such as higher sea-surface temperatures and reduced mixing depths that decrease nutrient concentrations in the upper layers of the ocean, go some way to explaining the decline. But biological processes may also be coming into play, for example a difference in the change in metabolic rates as temperatures rise of phytoplankton and the zooplankton that feed on them.
Now, a team from Germany, Switzerland, Canada and the US has modelled the chlorophyll changes using the POTSMOM-C ocean-atmosphere-sea-ice biogeochemistry model under the A1FI scenario for 1800–2100, and various increases in metabolic rate for zooplankton.
The team's simulations could reproduce about one quarter of the observed 20th century chlorophyll(a) decline when using a temperature coefficient (Q10) of zooplankton metabolism between 2 and 4, and a Q10 for phytoplankton growth of roughly 1.9.
Ocean warming could result in a relative boost in zooplankton compared to phytoplankton as respiration may be more temperature-dependent than photosynthesis.
The researchers recalibrated the standard ecosystem model, which had assumed a Q10 for zooplankton grazing of roughly 1.1. This led to a projection of a decline in global chlorophyll(a) of more than half by the end of the 21st century.
The model projected a rise of 6.9 °C in mean sea-surface temperature by 2100. For a zooplankton Q10 of 1.1, the team projected a 22.5% drop in chlorophyll by 2100. A zooplankton Q10 of 2 indicated a 48.1% decrease in chlorophyll, a Q10 of 3 gave a 54.4% reduction and a Q10 of 4 resulted in a 51% drop.
"A realistically parameterized model of zooplankton metabolism and phytoplankton growth can explain a large part of [the] observed global change in phytoplankton chlorophyll, indicating significant current and future declines," Dirk Olonscheck of the Potsdam Institute for Climate Impact Research, Germany, and ETH Zurich in Switzerland told environmentalresearchweb. "Furthermore [our results] indicate that both the marine net primary production and zooplankton biomass are much less sensitive to warming than the standing stock of phytoplankton."
Olonscheck believes this is likely to be due to a faster microbial turnover with higher temperatures. "This means that lower plankton biomass might not impact fisheries and biogeochemical cycling as drastically as one might think," he said.
Phytoplankton net primary productivity was projected to decrease 15–20% by 2100. Zooplankton, meanwhile, were projected to decline by about 6%, probably because of the reduction in the phytoplankton they feed on.
"Although the projected strong declines in phytoplankton Chla are projected to be partly compensated by an increased turnover under higher temperatures, the decline in plankton biomass constitutes a profound change in the future structure of marine ecosystems," said Olonscheck.
The team says that more realistic values of the temperature sensitivity of zooplankton metabolism should be used in future ocean-model simulations. "As our results have shown only slight deviations within the broad and likely range of Q10 = 2.0 – 4.0 for zooplankton metabolism, a value within this range seems to be appropriate," said Olonscheck. "Nevertheless, to finally confirm being within this range, comprehensive experimental research is needed to determine a reliable globally and species averaged Q10 parameter."
The scientists, who reported the results in Environmental Research Letters (ERL) , are also interested in tracing the wider ecological effects of changes in phytoplankton on fish stock productivity.
- Decomposing the effects of ocean warming on chlorophyll a concentrations into physically and biologically driven contributions, D Olonscheck, M Hofmann, B Worm and H J Schellnhuber 2013 Environ. Res. Lett. 8 014043
About the author
Liz Kalaugher is editor of environmentalresearchweb.