Matrix models for Arctic Calanus species: using experimental toxicity data for oil components to assess population level effects

When assessing population level effects of oil spills, data are required on
both the life history parameters of species and toxicity end-points. We
constructed age-structured matrix models for Arctic copepods to predict
the impact of potential oil spills at the population level. Two matrix models
were defined, representing the Arctic shelf species Calanus glacialis,
spawning in waters all around the Arctic shelf and C. hyperboreus, the
Arctic oceanic species connected to deep-sea, with a 2x2 and a 4x4 matrix
model, respectively, which refer to a 2-year and a 4-year life-cycle. The
post-breeding models included egg production and mortality rates as lifehistory
parameters. Relevant Arctic conditions were taken into account by
defining a winter diapause period, where no mortality is assumed to take
place. Toxicity information on exposure to oil and oil-components was
collected for Calanus species. It appeared that the life-history parameter
with the highest elasticity in the model was the adult survival, meaning that
changes therein have most impact on the output, i.e. population size. This
is because the adult class produces offspring. Published data on mortality
rates in field and laboratory studies appeared too high to maintain viable
populations. Therefore, we modified these parameter values such that modeled
populations became viable. Adequate toxicity information on oil and
oil components for Calanus species appeared to be scarce, and we therefore
selected pyrene as a model compound. However, reported end points for
pyrene were not directly applicable in the matrix model, and we developed
new approaches to derive a dose-response relationship from the available
data. Based on the generic matrix models, simulation models using day-today
calculation steps were developed to test the effects of various oil spill
scenarios at the population level, taking into account a range of exposure
concentrations and exposure durations. The results show that the calculated
population effects are very different, when based on different types
of effect concentrations (NOEC, LOEC and LC50). We argue that simple
matrix models are very useful to assess the potential impacts of oil spills
at the ecological relevant population level, but that scarcity of information
limits an accurate quantitative assessment. A more realistic estimate of
natural survival rates and the toxic effects thereupon will allow for a better
assessment of population level consequences of an oil spill.