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Landscape determinants of lake benthic and pelagic primary production

Citation

Puts, Isolde Callisto (2022), Landscape determinants of lake benthic and pelagic primary production, Dryad, Dataset, https://doi.org/10.5061/dryad.vx0k6djvs

Abstract

Global change affects gross primary production (GPP) in benthic and pelagic habitats of northern lakes by influencing catchment characteristics and lake water biogeochemistry. However, how changes in key environmental drivers manifest and impact total (i.e., benthic + pelagic) GPP and the partitioning of total GPP between habitats, here represented by the benthic share (autotrophic structuring) is unclear. This dataset presentes compiled data on summer gross primary productivity (GPP) in benthic and pelagic habitats sampled in situ between 2005-2017, together with water chemistry, from 26 shallow lakes (maximum 3.7-15.8 m deep) from three different sites in northern Sweden, located in the Arctic (Norrbotten), subarctic (Jämtland) and boreal (Västerbotten) biomes. The three study regions have variable elevation gradients and vegetation cover. The study lakes cover a wide range of DOC (1.5-16.3 mg·L-1) and accompanied water physico-chemistry.

Using this dataset, we investigate how catchment properties (air temperature, land cover, hydrology) affect lake physico-chemistry and patterns of total GPP and autotrophic structuring. We find that total GPP was mostly light limited, due to high dissolved organic carbon (DOC) concentrations originating from catchment soils with coniferous vegetation and wetlands, which is further promoted by high catchment runoff. In contrast, autotrophic structuring related mostly to the relative size of the benthic habitat, and was potentially modified by CO2 fertilization. Across Arctic and subarctic sites, DIC and CO2 were unrelated to DOC, indicating that external inputs of inorganic carbon can influence lake productivity patterns independent of terrestrial DOC supply. By comparison, DOC and CO2 were correlated across boreal lakes, suggesting that DOC mineralization acts as an important CO2 source for these sites.

Our results underline that GPP as a resource is regulated by landscape properties, and is sensitive to large-scale global changes (warming, hydrological intensification, recovery of acidification) that promote changes in catchment characteristics and aquatic physico-chemistry. Our findings aid in predicting global change impacts on autotrophic structuring, and thus community structure and resource use of aquatic consumers in general. Given the similarities of global changes across the Northern hemisphere, our findings are likely relevant for northern lakes globally. 

Methods

Database accompanying arcticle "Landscape determinants of pelagic and benthic primary production in northern lakes" with pelagic and benthic GPPz rates, and GPPlake-average with physico-chemical data for each lake. 

Sampling, lake water physico-chemistry and bathymetry

Water chemistry, PAR and temperature were measured on the same dates as GPP measurements. PAR and temperature were measured from the surface to the bottom with 1m depth intervals at the deepest part of the lake, with additional measurements at 0.25m and 0.5m using a handheld probe. Light attenuation coefficients (Kd) of the lake water were calculated as the absolute slope of natural logarithmically transformed photosynthetically active radiation (PAR) against depth. The sum of incoming PAR over the day was retrieved from stations we installed next to the lake. We used the water temperature measured at 0.2m depth (Twater) as proxy for lake epilimnion temperatures. Average air temperatures one month before sampling (Tair) were retrieved from weather stations (extracted from https://www.smhi.se) situated closest (within a range of 60km) to the sampling sites, and we included a temperature decrease of 0.57°C per 100m elevation difference between station and sampling site (sensu Karlsson et al., 2005 and references therein). Water samples for measuring pH, dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), total nitrogen (TN) and phosphorus (TP) were taken at 1m depth (epilimnion), or in nine cases from composite water samples (unstratified lakes). pH was measured directly after sampling and CO2 concentrations in the lake water were calculated from DIC, pH and temperature . In brief, DOC was filtered through a 0.45µm filter (Sarstedt Filtropur), acidified with HCl to an end concentration of 12 mM, and stored in a refrigerator before analyzed. TN and TP (unfiltered) samples were kept frozen until analysis. The DIC concentration was calculated from the headspace CO2 concentration in closed vials containing acidified lake water according to Åberg et al. (2007), and were analyzed as soon as possible. More details and specific lab operating procedures afterwards can be found in Appendix S1 accompanying the msnuscript. Detailed lake bathymetry was acquired through integrated GPS and echo-sounding depth measurements, from which we calculated lake average depth (zavg), lake volumes and areas (as a whole, or in different sections), as well as the relative areal size of the littoral benthic habitat (%Alittoral).

Gross primary production (GPP)

GPP was measured between 21 June and 28 July in variable years between 2005-2017 in situ in the benthic and pelagic on the same date. Benthic GPP was measured using the Dome-method (subarctic) or the DIC-method (Arctic and boreal). For the DIC-method, intact sediment cores with overlaying water were collected in incubation tubes using a gravity corer on three or five depths, and incubated for about 24h at the depth of collection. GPP rates at the discrete depths were measured by tracking changes in DIC concentrations between the onset and end of the incubation period in sealed off dark (respiration (R)) or transparent (R+GPP) tubes. In the Dome-method, three transparent domes equipped with a miniDOT oxygen logger were gently placed on the sediment at a different depth each, and O2 metabolism of the separated sediment area was measured for 24 hours. Benthic GPP rates at the three discrete depths were derived from net ecosystem production (NEP) using the R-package Lake Metabolizer (Winslow et al., 2016) and by assuming that GPP equals NEP plus R (by assuming R is oxygen loss during dark period; 24h metabolism). Pelagic GPP was measured at the surface, 0.25m, 0.5m and at following 1m depth intervals, where the deepest measurement depended on the lake depth and water turbidity. Measurements were done by incubating transparent glass bottles in situ filled with water from the sampling depth, with additional incubations in dark bottles at the most shallow and deepest measurements, for about four hours around noon using a 14C isotopic tracer. The GPP values measured for 4 hours midday at varying depths were converted to daily values by relating the midday measurements to the ratio of incident PAR during incubation time in relation to the daily PAR (24h). We used averages for duplicate or triplicate measurements of pelagic and benthic rates. An average lake GPP (mg C·m-2·day-1) was calculated for the benthic (benthic GPPlake-average) and pelagic (pelagic GPPlake-average) habitat. Benthic and pelagic GPP daily rates at discrete depths were upscaled to a lake average per m2 (benthic- and pelagic GPPlake-average; mg C·m-2·day-1) by integrating the GPP rates over the corresponding lake surface (benthic) or lake volume (pelagic) per depth interval, and relating the sum to the total lake area. The total average GPP of the lake (total GPPlake-average) is expressed as the sum of benthic and pelagic GPPlake-average (mg C·m-2·day-1), and autotrophic structuring is expressed as the relative amount (%) of GPP that takes place in the benthic habitat.

Funding

Svenska Forskningsrådet Formas, Award: 2016.00486

Knut och Alice Wallenbergs Stiftelse, Award: 2016.0083