Arctic Station Dirigibile Italia

Isotopic Characterisation of Arctic Ponds (ISOPOND)

Call year: 2015 Status: active IADC_id: 29

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SCIENTIFIC CONTEXT: Over the past three decades, recent warming altered biogeochemical and hydrological cycles across the Arctic, and the future temperature increase (2-2.4 °C according to current global circulation models) will continue to affect water and nutrient cycling in this region,  including reduced sea-ice extent, reduced snow cover and increased ice melting on land, anticipated breaking up and delayed freezing of ice-cover, and altered transportation of nutrients between terrestrial, freshwater an marine environments (Post et al. 2009, 2013). Despite the magnitude and the rate of abiotic changes reported in the Arctic, the ecological consequences of climate change remain relatively unreported (A.C.I.A. 2005). Arctic ponds represent a very good laboratory and compelling models to study the short time effect of global warming, and can be considered “sentinels” of climate changes (Vincent et al. 2008). Indeed, ponds may be subjected to rapid and nonlinear ecological changes following an increase in temperature and nutrient inputs, given the generally nutrient limiting conditions and the short time-window available for productivity (i.e. the ice-free period) (Prowse et al. 2006). Physical and biological conditions in ponds are tightly coupled with the freezing and melt cycle of ice, which will be profoundly affected by climate warming. In addition, their high dependency by the surrounding environment and external inputs (e.g. nutrient and organic matter from terrestrial vegetation and guano deposition from birds where present) makes these systems subjected to multiple forcing factors that control the productivity and the structure of biological communities, resulting in a high sensitivity to alterations related to nutrient loading and climate change (Hobbie et al. 1999). Such changes are expected to be more pronounced in islands with respect to the mainland, given the sea-ice loss-related amplification of arctic warming in costal environments (Post et al. 2013). In Svalbard, the climate can be characterized as “arctic semi-desert”. In this severely nutrient-limited habitat, ponds represent hotspots of biodiversity and biological activity, being key elements acting as nutrient and biomass sink/source for terrestrial and coastal environments, respectively. Terrestrial vegetation cover and diversity, water inputs, ice-cover dynamics and landscape heterogeneity are likely to control the inputs of nutrients from catchment areas (Vincent et al. 2008). Both abiotic and biotic features in ponds and in their catchments result in extremely low rates of nutrient delivery in the water bodies, where productivity is primarily limited by nutrient supply, low temperature, and short period of light availability due to ice cover (Hobbie et al. 1999, Vincent et al. 2008). Nutrients (particularly, C, N and P) are mainly transported in the organic form, through movement of dissolved and particulate organic matter originating from land (Kling 1995). Ecosystem productivity and biomass in arctic ponds is dominated by the benthic photosynthetic community, where nutrient cycling in sediments enhance productivity of aquatic plants living in the bottom with respect to the phytoplankton in the water column (Hobbie et al., 1999; Vincent et al. 2008). A study from a tundra lake (Canada) showed that benthic primary productivity accounted for around the half of the organic carbon available to aquatic consumers, while organic detritus from terrestrial vegetation and phytoplankton accounted for the remaining 30% and 20%, respectively (Ramlal et al., 1994). In turn, benthic productivity is more likely to be limited by light availability due to ice cover, and climate changes are expected to alter profoundly the relative importance of benthic and planktonic compartments for the biological production (Smol and Douglas 2007). Phytoplankton community structure and clorophill levels in water have been found to be influenced by dissolved nutrient availability, temperature and basin shape and depth (Pienitz et al., 1995; Vincent et al., 2008). In its turn, organic nutrient flux has been shown to vary according to pond position and terrestrial vegetation cover (Kling, 1995). Considering all this, heterogeneity in catchment features and dependency by external forcing factors of Svalbard ponds should be expected to produce differences in their ecological response to climate changes (Vincent et al., 2008; Post et al., 2009). Nevertheless, little attention has been paid to understand factors linking such heterogeneity in pond and landscape characteristics and control mechanisms dominating both allochthonous and autochthonous nutrient and organic matter inputs. Quantifying these controls is needed as a first fundamental step to understand what combination of factors subtend the ecological equilibrium and productivity in arctic ponds and their future response to climate change. By mean of elemental and isotopic analyses of carbon and nitrogen as tracers of nutrient and organic matter inputs (Rossi et al. 2010; Calizza et al. 2012, 2013; Costantini, Calizza and Rossi, 2014; Orlandi et al., 2014; Careddu et al., 2015) in Svalbard ponds and their catchments, this pilot research project will provide key information to quantify the relative importance of autochthonous and allochthonous inputs subtending to the biological production in arctic ponds.rnRESEARCH DESCRIPTION: Ponds are one of the most abundant ecosystem types in the Arctic. On Svalbard, they form a “mosaic of water bodies” that represent hotspots of ecological diversity in the tundra, providing diverse aquatic habitats for the biological community, including microbes, benthic invertebrates, plants, plankton, fish and birds. Five major factors are likely to control nutrient and organic matter inputs in the water body, all of which will be importantly affected by climate warming and associated ecological changes (Vincent et al. 2008, Post et al. 2009). (1) Ice cover, whose presence limits wind-induced vertical mixing of waters, the redistribution of nutrients from sediments in the water column, and light availability on the bottom. Thus, reduction in thickness and duration of ice cover associated to climate warming may promote both algal and phytoplankton blooms. (2) Snow and ice run-off after melting from the watershed, which transports dissolved nutrients and particulate detritus deriving from plants within the catchment area, and which is expected to increase following warming and the increase of rain in precipitation. (3) Terrestrial vegetation, which ultimately controls the production of allochthonous nutrients and organic matter and affects the presence and abundance of aquatic birds in the watershed (Vincent et al. 2008, Post et al. 2009). Warming is expected to promote the expansion and productivity of graminoids, low-arctic trees and shrubs (Tape et al. 2006, Dandy and Hik 2007), with fundamental implication in nutrient fluxes both at the catchment and the landscape scale in Svalbard. (4) Guano deposition from birds (mainly geese), which represent an important subsidiary energy input boosting productivity in these nutrient-limited environments (Prop et al. 1984, Luoto et al. 2014). At Svalbard, geese populations increased over the last decades, in association with both increased food availability and reduced winter mortality due to less severe climatic conditions, representing a key factor linking terrestrial and aquatic productivity (Smol and Douglas 2007, Van Geest et al. 2007). (5) Biological control of primary productivity through food web mediated effects. Phytoplankton consumption by zooplankton (mainly Daphnia spp.) can be intense in fishless ponds, which may limit algal blooms also when in conditions of additional nutrient inputs. On the other hand, a study in a pond from Svalbard showed that Daphnia was subsidized by important energy inputs from benthic trophic sources, trapping nutrients potentially available to phytoplankton following sediment-water mixing. Heterogeneity in pond and watershed characteristics, including pond morphometry and position, ice-cover, vegetation in the watershed, will determine the relative importance of each of these factors in the control on the allochthonous and autochthonous nutrient and organic matter inputs in the water bodies. Quantifying these controls is a first necessary step to understand what combination of factors subtend the ecological equilibrium and productivity in arctic ponds and will drive their ecological response to climate changes.rnDetermining the relative contribution of different allochthonous and autochthonous inputs to organic deposits and dissolved nutrients in aquatic systems is not easy task (Rossi et al. 2010). Indeed, diverse organic inputs result in complex mixtures accumulating on the bottom, often lacking of physical distinction (Moran and Hodson 1989, Rossi et al. 1990), and physic-chemical analyses in waters can’t elucidate the origin of dissolved nutrients. Stable isotope and elemental analyses of carbon (?13C) and nitrogen (?15N) represent efficient tools to the identification of the relative contributions of various elemental sources to nutrients assimilated by primary producers and benthic organic deposits, and thus their contribution to higher trophic levels through the food web (Rossi et al., 2010; Calizza et al., 2012, 2013; Costantini, Calizza and Rossi, 2014; Orlandi et al., 2014; Careddu et al., 2015). Indeed, ?13C  have been shown to vary between terrestrial and aquatic primary producers, differing between C3 and C4 plants on land and between benthic algae and phytoplankton in waters (e.g. Calizza et al. 2012, 2013; Careddu et al., 2015). On the other hand, ?15N can be used to investigate nutrient loading pathway in aquatic systems (Costanzo et al, 2001; Orlandi et. al., 2014), with organic N being characterized by higher ?15N values with respect to inorganic-derived nitrogen in biological samples (Costanzo et al, 2001; Orlandi et. al., 2014, Careddu et al. 2015). ?15N has been shown to undergo a predictable stepwise increase with trophic position, being useful to discriminate between vegetal and animal tissues and their contribution to the organic position in sediments (Kling et al., 1992; Post et al., 2002; Careddu et al. 2015). In combination with isotopic analyses, the elemental content of C and N and their ratio in samples can further improve the unambiguous solution of an isotopic mix when sources partially overlap in their isotopic signatures, as both vegetal and animal tissues can significantly differ in their relative content of elements (Yuan et al., 2009). Basing on the isotopic signature of carbon (?13C) and nitrogen (?15N) in (i) organic mixture from sediments and in both benthic and planktonic primary producers, and (ii) the potential autochthonous and allochthonous elemental sources (including vegetation in the watershed, organic matter in soil and sediments, fecal deposition from birds, and aquatic primary producers as potential contributors of organic matter in sediments), Bayesian mixing models (Parnell et al. 2010, 2011) will be applied to isotopic data belonging to ponds in Spitsbergen (Svalbard) differing in their position, surface area and catchment features, in order to obtain probability distributions of the relative contribution of each potential source to both organic matter in sediments and nutrients assimilated by benthic and planktonic primary producers in each pond. The elemental composition (i.e. the percentage of C and N in samples) will be considered to further discriminate sources partially overlapping in their isotopic signatures (Yuan et al. 2009), thus providing a triple (?13C, ?15N, and C to N ratio) biochemical characterisation of samples and achieving an elevated resolutive power of Bayesian isotopic mixing models. In addition, the inclusion of C and N content of sources in mixing models allows to weight their isotopic contribution to the mix for their relative difference in elemental content (Phillips and Koch 2002). By mean of elemental and isotopic analyses of carbon and nitrogen as tracers of nutrient and organic matter inputs in Svalbard ponds and their catchments, this pilot research project aims at the quantification of the relative importance of autochthonous and allochthonous inputs subtending to nutrient stocks and biological production in arctic ponds, and their relationship with pond morphometry and position, ice-cover, and vegetation in the watershed, representing a valuable baseline to understand and anticipate future ecological responses to climate changes in these unique and vulnerable habitats. This question is not only of interest in polar freshwater habitats, having the potential to provide mechanistic understanding into the controlling factors of biological productivity at other latitudes, improving our predictive power on how aquatic ecosystems will respond to future climate change.rnOBJECTIVES:(i) To determine the C (?13C) and N (?15N) isotopic signatures in sediments, benthic and planktonic primary producers, terrestrial vegetation in the watershed, soil, and fecal deposition from birds in the watershed of each pond considered in the study. This will produce an unprecedented isotopic dataset for pond catchments in Svalbard, which will also represent an important baseline for future studies aiming at the description of the trophic niche of terrestrial and aquatic consumers by mean of the isotopic analysis of their potential food sources. (ii) To determine the relative elemental content in C and N in sediments, benthic and planktonic primary producers, terrestrial vegetation in the watershed, soil, and fecal deposition from birds in the watershed of each pond considered in the study. This will allow to further discriminate sources partially overlapping in their isotopic signatures, thus providing a triple biochemical characterisation of samples and achieving an elevated resolutive power of Bayesian isotopic mixing models. In addition, the quantification of C to N ratio and the percent of ash-free dry matter content in aquatic sediments will provide a relative measure of the quality and the amount of organic nutrients available for biological productivity (Calizza et al., 2012; Costantini, Calizza, and Rossi, 2014). (iii) To obtain isotopic evidences and Bayesian isotopic mixing model outputs quantifying the relative contribution of each potential source (including vegetation in the watershed, organic matter in soil, fecal deposition from birds, and aquatic primary producers as potential contributors of organic matter in sediments) to both nutrients assimilated by benthic and planktonic producers and organic matter deposition in sediments. (iv) To relate inter-pond differences in the isotopic signature of aquatic producers and Bayesian isotopic mixing models outputs for sediments with differences in pond morphometry, position, ice-cover, terrestrial vegetation in the watershed, and fecal deposition by aquatic birds. The comprehension of the relative importance of each of these factors in the control of nutrient and organic matter inputs in ponds will allow to advance in the comprehension of the ecological dynamics characterizing these systems both at the catchment and the landscape scale in Svalbard.rnMETHODOLOGY: The research is based on the elemental and isotopic analysis of carbon and nitrogen in sediments, benthic and planktonic primary producers, terrestrial vegetation in the watershed, soil, and guano from birds in the watershed of freshwater ponds in Spitsbergen (Svalbard), in the area of Ny-Alesund. Samples will be collected in ponds differing in their morphometry, position, ice-cover, terrestrial vegetation, and fecal deposition by aquatic birds in the watershed. In addition, the percentage of ash-free dry matter content in samples will be quantified. The stable isotope signatures (?13C e ?15N) and relative C and N content of samples will be determined in a Elementar vario-MICRO CUBE analyser (Elementar, Hanau, Germany) coupled with an Isoprime 100 mass spectrometer (Isoprime Limited, Cheadle Hulme, UK), operating as a continuous flow system. Outputs will be standardized with caffeine. International reference standards will be PeeDee Belemnite carbonate for ?13C and atmospheric N2 for ?15N (Peterson and Fry, 1987). Analyses will be performed at the Laboratory of Trophic Ecology, Department of Environmental Biology, Sapienza University of Rome (Rome, Italy). Samples will be collected in Svalbard and stored (-20 °C or oven dried, depending by sample typology) for transportation in Italy. A Bayesian isotopic mixing model available as an open source R package (SIAR, Stable Isotope Analysis in R) will be used to assess the relative contributions of potential nutrient and organic matter sources to pond sediments and both benthic and planktonic primary producers (Parnell et al., 2010, Calizza et al. 2013). Sediments will be separated by sieving in coarse (> 1 mm), fine (between 1 mm and 0.056 mm) and ultra fine (< 0.056 mm) fraction. Each fraction will be dried at 60 °C for 72 h and weighed to assess dry weight. The ash-free dry organic matter content will be then assessed after muffle combustion (5 h at 500 °C) (Calizza et al. 2012).

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Isotopic Characterisation of Arctic Ponds (ISOPOND)