Spatial variability of Arctic sea ice algae

Hauke.Flores [ at ]


The most pronounced effects of global climate change have been experienced in the Arctic region. In particular, Arctic sea ice decline and volume loss have emphasized the impeding threat of continued climate change, and have been center stage in the public eye for over a decade. Many of the observed changes in the Arctic are related to the physical system because these parameters, such as sea ice extent and thickness, are more easily observed from space and airborne platforms. The linkage between ecosystem function and its physical environment is clear from all well investigated systems. This undoubtedly means that the observed changes to the physical system have had an equally dramatic impact on the Arctic ecosystem. Our understanding of the Arctic marine ecosystem, however, is severely limited due to the methodological and logistical constraints of monitoring ecological properties. This has caused significant seasonal and geographical knowledge gaps, particularly in the high (> 80ºN) and central Arctic Ocean. Over the past decades a disproportional emphasis has been put on the importance of primary production (PP) and the availability of food in the water column. Observations have indicated an overall increase in Arctic-wide net primary production (NPP) as a result of a thinning and declining sea ice cover, and increasing duration of the phytoplankton growth season. This increased biomass may suggest a corresponding increase in the biomass of consumers and higher trophic levels. This premise, however, neglects the rather important role that the sea ice environment and sea ice algae play in the Arctic food web. The timing, duration and spatial availability of ice algae are drastically different compared to pelagic phytoplankton. Therefore, it is only by first gaining a better understanding of the base of the Arctic food web that we can start to understand the rest of the food web. Throughout this thesis, we aimed to assess how sea ice algae biomass availability and habitat will be affected by continued changes to the sea ice habitat, and what consequences can be expected for the Arctic food web. This was accomplished by developing novel methodologies and approaches to characterize and quantify the spatial variability of sea ice algae-biomass, -primary production and – habitat. Subsequently, we used this toolset to assess the implications of a rapidly changing sea ice habitat in relation to spatial variability of sea ice algae carbon availability and carbon demand by iceassociated organisms. In Chapter 2, we developed a methodological toolbox to process environmental sensor array observations acquired from under-ice profiling platforms (e.g., Remotely Operated Vehicle – ROV, and the Surface and Under-Ice Trawl – SUIT), which included novel mathematical and statistical approaches to representatively capture the spatial variability of sea ice and under-ice physicalbiological properties. We showed that our developed approaches produced observations, which could capture the spatial variability better than traditional point location characterizations of environmental properties. Specifically, the insufficient spatial representativeness of sea ice-algal biomass can cause biases in large-scale ice algal biomass and PP estimates. In Chapter 3, we further developed upon Chapter 2 methodologies by introducing a new approach to estimate primary production on floe-scales (meters to kilometers), further justifying the need for representative ice-algae biomass and PP estimates. We also showed that the sea ice environment and under-ice water properties played an important role in structuring the under-ice community. Furthermore, we indicated that ecological key species of the central Arctic Ocean thrived significantly on carbon synthesized by ice algae. These results highlighted the key role of sea ice as a habitat and as a feeding ground within the Arctic Ocean. In Chapter 4, we aimed to compare the physical-biological properties of multi-year sea ice (MYI) and first-year sea ice (FYI) to provide some insight into how the Arctic will change with the continued replacement of MYI by FYI. We developed and confirmed the hypothesis that thick MYI hummocks do have the potential to host substantial ice algae biomass and identified hummocks as common and permanent features, which represent a reliable habitat for sea ice algae due to the typically thin or absent snow cover. We developed key physical-biological relationships to classify the springtime spatial variability of sea ice algae habitat for both FYI and MYI. We applied this classification to pan- Arctic ice thickness and snow observations, and showed that MYI is substantially under-estimated in terms of suitable habitat. Furthermore, we identified thick sea ice features, such as MYI hummocks and sea ice ridges, as potentially high biomass regions with great ecological value. We also indicated that the thicker sea ice, which remains in late-summer, has reduced melt-induced algal losses. In conclusion, we developed a robust and novel approach to representatively quantify sea ice environmental properties, and sea ice algae biomass and PP at floe-scales. These estimates resulted in more accurate estimates of overall carbon biomass availability and production, which we used to improve the spatial variability of the ice-algae derived carbon budget. We concluded that there was a large mis-match between ice-algal primary produced carbon and ice-algal carbon demand by dominant species. This mis-match was also accompanied by large regional variability. This was expected during our sampling period since production was shutting down. Taking a different approach, we showed that the standing stocks of ice-algal carbon were quite substantial. These results suggest that during late-summer, when primary production shuts down, the remaining ice-algal biomass in high latitude regions may represent a crucial food source to sustain ice associated organisms during the onset of polar night. Altogether, the continued thinning and loss of thicker sea ice features may result in the loss of a reliable carbon supply, in the form of sea ice algae carbon, at key times of the year when other carbon sources are severely limited.

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Lange, B. (2016): Spatial variability of Arctic sea ice algae , PhD thesis, University of Hamburg.

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