Baumeister, Moritz (2017) The impact of fluctuating temperature regimes on the common starfish Asterias rubens. (Bachelor thesis), Universität Oldenburg, Oldenburg, Germany, 40 pp.

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Abstract

Global climate change already influences the oceans’ ecosystems today. This apparent impact will intensify in the future due to pronounced alterations of abiotic conditions. Particularly warmer sea surface temperatures are considered as a major driver with strong implications from species to ecosystems. Over the past decades, most of the experimental studies focused on mean shifts of stressors and thus intentionally neglected the role of natural environmental variability within systems. Recently, however, scientists highlighted the importance of fluctuating stress regimes as a driver of species responses. In constant stress regimes, potential phases of recovery in low-stress periods (e.g. colder temperatures) as well as very extreme stress or even lethal stress in peak periods (e.g. heat waves) are, however, ignored and thus providing limited relevance for predicting future ecosystem changes. In the present study, a predator-prey system was investigated under stress from constant and fluctuating temperature regimes. A sinusoidal fluctuation mode of temperature with a wavelength of eight days was applied. We compared this fluctuation mode to a temperature regime in a constant mode, however, both treatments experienced equal mean temperatures. Constant and fluctuating treatments were applied in today`s temperatures and in temperature projected for the end of this century, taking a seasonal pattern of a temperature regime from July to September into account. Within this 72 days experiment, we investigated the common starfish Asterias rubens (hereafter Asterias), which is a keystone predator in the Western Baltic Sea. The main responses towards the different temperature treatments were investigated as feeding behaviour on the blue mussel Mytilus edulis (hereafter Mytilus), as well as by measuring overall biomass increment and righting response as a measure of fitness. Asterias significantly decreased feeding on Mytilus under future warming conditions, regardless of fluctuating or constant mode, and thus their energy accumulation and overall growth was reduced. Notably, fluctuations around future means imposed an additional stress on Asterias leading to a significantly earlier collapse in feeding under fluctuating than under constant conditions. Here, the first two temperature peaks impacted Asterias, so these individuals did not start feeding afterwards. Additionally, possibly resulting from accumulative stress, Asterias was not capable of feeding towards the end of the experiment, although here, temperatures decreased to tolerable conditions due to seasonality (in particular in the constant treatment). At todays’ mean conditions, fluctuations led to a, yet not statistically significant, trend of lower performance when compared to the constant temperature regime. Here, Asterias feeding on Mytilus was largely reduced during peak temperatures and not entirely compensated during phases of stress release. Particularly, the future fluctuating temperature regime led to a strongly decreased fitness (increased time of righting of Asterias individuals). Overall, in a warmed future ocean, Asterias will likely lose its central role as key predator on mytilid mussels due to its strong sensitivity to temperature stress. The potentially decreased feeding pressure on Mytilus can therefore reshape the entire currently Mytilusdominated Western Baltic Sea ecosystem. Asterias might, however, be – entirely or partly – replaced by other established (Carcinus maenas) or invasive (Hemigrapsus takanoi) predators with lower temperature sensitivities. One crucial aspect of future research will be the identification of the role of environmental fluctuations in a climate change context. This study suggests that the negative impacts from warmer sea surface temperatures are rather amplified and that phases of recovery might not compensate for phases of increased stress.

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