Warming effects on marine microbial food web processes: how far can we go when it comes to predictions?

Abstract

Previsions of a warmer ocean as a consequence of climatic change point to a 2-68C temperature rise during this century in surface oceanic waters. Heterotrophic bacteria occupy the central position of the marine microbial food web, and their metabolic activity and interactions with other compartments within the web are regulated by temperature. In particular, key ecosystem processes like bacterial production (BP), respiration (BR), growth efficiency and bacterial -grazer trophic interactions are likely to change in a warmer ocean. Different approaches can be used to predict these changes. Here we combine evidence of the effects of temperature on these processes and interactions coming from laboratory experiments, space-for-time substitutions, long-term data from microbial observatories and theoretical predictions. Some of the evidence we gathered shows opposite trends to warming depending on the spatio-temporal scale of observation, and the complexity of the system under study. In particular, we show that warming (i) increases BR, (ii) increases bacterial losses to their grazers, and thus bacterial -grazer biomass flux within the microbial food web, (iii) increases BP if enough resources are available (as labile organic matter derived from phytoplankton excretion or lysis), and (iv) increases bacterial losses to grazing at lower rates than BP, and hence decreasing the proportion of production removed by grazers. As a consequence, bacterial abundance would also increase and reinforce the already dominant role of microbes in the carbon cycle of a warmer ocean.

Climate change, biotic interactions and ecosystem services

“…Two studies presented in this volume (Sarmento et al 2010;Yvon-Durocher et al 2010) suggest a different scenario. In them, ecosystem processes are affected by climate change independently of changes in biodiversity or biotic interactions.…”

“…Or it can provide the baseline of process rates in more dynamic ecosystems, addressing how ecosystem processes would change owing to warming, other things being equal. Petchey et al (2010), Woodward et al (2010), Yvon-Durocher et al (2010) and Sarmento et al (2010) apply and extend this theory to predict changes in specific ecosystem processes. They show that it works in most cases and that it provides accurate quantitative predictions.…”

“…This can be simply explained by differences in the sensitivity of both processes to temperature changes, independently of changes in other ecosystem attributes like standing biomass, community composition or trophic interactions. Sarmento et al (2010) also use the metabolic theory of ecology, in this case to predict changes in marine microbial food-web processes. They test several theoretical predictions with evidence from laboratory experiments, space-for-time substitutions and longterm data from microbial observatories.…”

Long-term effects of ocean warming on the prokaryotic community: evidence from the vibrios

“…Although much evidence has been accumulated on the long-term effects of ocean warming on eukaryotic populations (for example, animals and plants, Edwards and Richardson, 2004;Sala and Knowlton, 2006), no experimental information exists for the effects this may have on marine prokaryotic abundance and diversity (Sarmento et al, 2010). An explanation for this gap is the lack of historical data and the belief that lower trophic levels, such as the primary producers (phytoplankton) and decomposers (heterotrophic prokaryotes), are considered less sensitive to environmental change than their consumers or predators, because sensitivity to climate is considered to increase with trophic level (Voigt et al, 2003;Raffaelli, 2004).…”

Status and trends in the structure of Arctic benthic food webs

“…Because of their short generation times, microorganisms respond quickly to environmental changes and are the first to react to global changes, affecting key ecosystem functions at the base of food webs (Sarmento et al 2010). Significant changes in microbial communities and diversity have already been observed after a sharp decline of sea-ice cover in September 2007 (between 2002 and2010;Comeau et al 2011).…”