Effects of mortality changes on biomass and production in Calanus spp. populations

Abstract

Calanus species are the main link between primary producers and higher trophic-level organisms in the Barents Sea. The natural mortality rate is an essential parameter for determining the standing stock of Calanus, but it is also one of the most uncertain parameters in present knowledge. The level of human activity, and the associated risk of pollution, is increasing in the Barents Sea, and knowledge of the Calanus population response to increased mortality is crucial for management of the ecosystem. In the present study, we estimated natural mortality rates of Calanus, based on available field data from the Barents Sea, and performed numerical simulation experiments with a coupled physical-biological model, testing the response of Calanus populations to changes in mortality rates, and other related ecological parameters. The field-based estimates of natural mortality showed high variability. The model simulations showed that the 2 Calanus species modelled, C. glacialis and C. finmarchicus, respond differently to increased mortality, and that in creased mortality alters both the timing of peak Calanus production and biomass relative to peak primary production. These simulations illustrate the potential for a mismatch between peak food availability and Calanus population dynamics in the Barents Sea as a consequence of natural or human-induced perturbations. We suggest that the observed differences in the 2 Calanus species' responses to perturbations relates to each species' life cycle and habitat characteristics. The present study illustrates how models can be used to assess key parameters affecting species' population dynamics and some potential consequence of external forcing factors affecting mortality. KEY WORDS: Zooplankton · Calanus finmarchicus · Calanus glacialis · Numerical modelling · Ecological modelling · Arctic · North Atlantic · Barent Sea · SimulationResale or republication not permitted without written consent of the publisher Aquat Biol 12: 129-145, 2011 130 Ohman 1996). Mortality may be the result of natural events driven by, for example, limited food availability or the presence of abundant predatory species, or by human activities, such as pollution. In the Arctic, industrial activities are on the rise. For example in the Barents Sea, petroleum transport from Russia along the Norwegian coastline increased from insignificant volumes in 2002 to an estimated 20 million tons in 2009 (Bambulyak & Frantzen 2009). Parts of the Barents Sea have already been opened for oil and gas exploration, and the Lofoten and Vesterålen shelves, at the 'entrance' to the Barents Sea, are currently being considered for oil and gas exploration by the Norwegian government. In creasing petro leum transport, oil and gas exploration, and other industrial activities increase the risk of accidents and the potential for negative impacts on individual organisms and ecosystems (Chap man & Riddle 2003). Understanding Calanus population re sponses to increased mortality is crucial for management of the Barents Sea ...

No evidence for hybridization between Calanus finmarchicus and Calanus glacialis in a subarctic area of sympatry

“…Likewise, the drop in Calanus spp. abundance observed from February to March in both fjords cannot be directly associated with an advective effect either because levels of mortality are expected to increase at that time (Skarðhamar et al 2011). Therefore, it seems that the four Calanus species recorded in our seasonal study spend a large part of the winter together in sympatry in the fjords.…”

Productivity in the Barents Sea - Response to Recent Climate Variability

“…As stated above, many previously published studies have shown that top-down control of zooplankton is a key process potentially regulating zooplankton biomass and hence production in the Barents Sea. However, a model study has shown that a reduction in overwintering biomass of zooplankton, may not necessarily influence the production in the following years [60] . In their study, the results differed between the two main Calanus species: while the overwintering biomass of C. finmarchicus did influence the next year’s production of this species, the opposite was seen for its Arctic counterpart C. glacialis .…”

Estimating mortality: Clarifying assumptions and sources of uncertainty in vertical methods

“…Vertical methods are relatively robust to moderate noise, and the literature advocates their use over horizontal methods when transport influences abundance, provided different development stages are equally influenced by advection (e.g. Aksnes and Ohman, 1996;Gislason et al, 2007;Hirst et al, 2007;Skarohamar et al, 2011). Their main limitation is cited as being sensitivity to temporal trends in recruitment Aksnes et al, 1997).…”

“…When time series data are available, mortality can be estimated by simulating the population dynamics, and adjusting modeled mortality rates until the model results give a good fit to the observations Neuheimer et al, 2009;2010a,b;Skarohamar et al, 2011). Alternatively, time series data can be used with the so-called horizontal estimation methods (Aksnes et al, 1997), for which formulae are derived from models of temporal changes to cohorts or "closed populations" caused by reproduction, growth and mortality.…”

“…Even small changes in mortality can have profound effects on copepod demography, dynamics and production (e.g. Lynch et al, 1998;Skarohamar et al, 2011;Speirs et al, 2006;Twombly, 1994). Consequently, accurate estimates of copepod mortality rates are essential for understanding copepod life history and trophic links, as well as predicting ecological responses to climate-related changes in the environment.…”