Abstract
Phytoplankton dynamics are closely linked to the ocean-climate system with evidence that changing ocean conditions are substantially altering phytoplankton biogeography, abundance and phenology. Here, using phytoplankton community composition and environmental data spanning 1965 to 2013 from a long-term Pacific Ocean coastal station offshore from Sydney, Australia (Port Hacking 100m), we used the Maximum Entropy Modelling framework (MaxEnt) to test whether phytoplankton realised niches are fixed or shift in response to changing environmental conditions. The mean niches of phytoplankton closely tracked changes in mean temperature, while the mean salinity and mixed layer depth niches were consistently at the extreme range of available conditions. Prior studies had shown a fixed niche for nitrate in some phytoplankton species at a site where nitrate concentration was decreasing and potentially limiting; however, at Port Hacking nitrate and silicate niches increased more rapidly than environmental conditions, apparently in response to periodic occurrences of elevated nutrient concentrations. This study provides further evidence that climate change model projections cannot assume fixed realised niches of biotic communities, whilst highlighting the importance of sustained ocean measurements from the southern hemisphere to enhance our understanding of global ocean trends.
Highlights
Warming of the Earth’s ocean and atmosphere due to anthropogenic CO2 emissions has seen a global average increase in surface air temperature of 0.85◦C over the past century (IPCC, 2014), with the upper ocean trapping the majority of the anthropogenic heating (IPCC, 2013)
Four environmental variables allowed comparisons across all three periods while comparisons for silicate concentration were only possible between the latter time periods P2–P3 (Table 2)
We found changes in the community average mean niches and the mean environmental conditions were always in the same direction for temperature, mixed-layer depth (MLD) and nitrate concentration between P1 and P2 and between P2 and P3
Summary
Warming of the Earth’s ocean and atmosphere due to anthropogenic CO2 emissions has seen a global average increase in surface air temperature of 0.85◦C over the past century (IPCC, 2014), with the upper ocean trapping the majority of the anthropogenic heating (IPCC, 2013). Atmospheric warming is associated with a decrease in pH and acidification of ocean waters (Rost et al, 2008; Beaufort et al, 2011). It is in these upper, sunlit waters of the global ocean that phytoplankton flourish, producing ∼ 45 Gt a−1 of organic carbon (Falkowski et al, 1998; Field et al, 1998). The flow-on effects that changes in climate and phytoplankton communities may have on ocean food webs and global biogeochemical cycles are poorly understood yet potentially profound and include the potential for harmful algal bloom intensification (Edwards and Richardson, 2004; Gobler et al, 2017). Establishing links between climate change and trends in the structure of phytoplankton assemblages is challenging, as phytoplankton have been shown to exhibit ordersof-magnitude variability over seasonal, inter-annual and interdecadal time scales (Zingone et al, 2010)
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