Abstract
Abstract. Iron availability and temperature are important limiting factors for the biota in many areas of the world ocean, and both have been predicted to change in future climate scenarios. However, the impacts of combined changes in these two key factors on microbial trophic dynamics and nutrient cycling are unknown. We examined the relative effects of iron addition (+1 nM) and increased temperature (+4°C) on plankton assemblages of the Ross Sea, Antarctica, a region characterized by annual algal blooms and an active microbial community. Increased iron and temperature individually had consistently significant but relatively minor positive effects on total phytoplankton abundance, phytoplankton and microzooplankton community composition, as well as photosynthetic parameters and nutrient drawdown. Unexpectedly, increased iron had a consistently negative impact on microzooplankton abundance, most likely a secondary response to changes in phytoplankton community composition. When iron and temperature were increased in concert, the resulting interactive effects were greatly magnified. This synergy between iron and temperature increases would not have been predictable by examining the effects of each variable individually. Our results suggest the possibility that if iron availability increases under future climate regimes, the impacts of predicted temperature increases on plankton assemblages in polar regions could be significantly enhanced. Such synergistic and antagonistic interactions between individual climate change variables highlight the importance of multivariate studies for marine global change experiments.
Highlights
The Ross Sea, Antarctica is the location of one of the world’s largest annual algal blooms (Smith et al, 2000)
Total chlorophyll and the >20 μm chlorophyll size fraction both increased in all treatments over the course of the experiment (Fig. 1)
Final day total chlorophyll was lowest in the control treatment, higher in the high temperature treatment, and higher still in the high iron treatment
Summary
The Ross Sea, Antarctica is the location of one of the world’s largest annual algal blooms (Smith et al, 2000). In view of the expected minimum global temperature rise of 1–3◦C over the century (Alley et al, 2007), the broad future trends in coastal Antarctic ecosystems seem likely to include further widespread warming and ice retreat If so, these changes in seasonal ice dynamics and accompanying increased stratification effects will lead to major changes in mixing regimes, nutrient supplies, and light environments experienced by Antarctic phytoplankton (Boyd and Doney, 2002; Montes-Hugo et al, 2009). Less is known for certain about future shifts in iron supplies to the iron-limited phytoplankton communities of the Ross Sea. Since the largest current sources of iron to surface waters here are vertical mixing and seasonal sea ice melting (Coale et al, 2003; Fitzwater et al, 2000; Martin et al, 1990; Sedwick and DiTullio, 1997; Sedwick et al, 2000), iron availability is highly vulnerable to anticipated climate change impacts.
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