DOI: 10.5061/dryad.r2280gbbq
title: Antagonistic effects of temperature and dissolved organic carbon on fish growth in California mountain lakes

description: Resources and temperature play major roles in determining biological
 production in lake ecosystems. Lakes have been warming and ‘browning’ over
 recent decades due to climate change and increased loading of terrestrial
 organic matter. Conflicting hypotheses and evidence have been presented
 about whether these changes will increase or decrease fish growth within
 lakes. Most studies have been conducted in low-elevation lakes where
 terrestrially derived carbon tends to dominate over carbon produced within
 lakes. Understanding how fish in high-elevation mountain lakes will
 respond to warming and browning is particularly needed as warming effects
 are magnified for mountain lakes and treeline is advancing to higher
 elevations. We sampled 21 trout populations in the Sierra Nevada Mountains
 of California to examine how body condition and individual growth rates,
 measured by otolith analysis, varied across independent elevational
 gradients in temperature and dissolved organic carbon (DOC). We found that
 fish grew faster at warmer temperatures and higher nitrogen (TN), but
 slower in high DOC lakes. Additionally, fish showed better body condition
 in lakes with higher TN, higher elevation and when they exhibited a more
 terrestrial δ13C isotopic signature. The future warming and browning of
 lakes will likely have antagonistic impacts on fish growth, reducing the
 predicted independent impact of warming and browning alone.
description: <strong>Lake sampling</strong> At
 the deepest point in each lake, in situ measurements of temperature was
 taken using a YSI probe (YSI Incorporated, Yellow Springs, Ohio, USA).
 Surface water samples were filtered through 63-μm mesh to remove
 zooplankton and processed for chlorophyll-a (chl-a), particulate organic
 matter (POM), total nitrogen (TN), total phosphorus (TP) and dissolved
 organic carbon (DOC). For chl-a quantification, a known volume of water
 was filtered through 0.7 μm glass fiber filters (GF/F Fisher Scientific)
 and frozen. Chl-a, a proxy for phytoplankton biomass, was measured using a
 fluorometer after a 24 h cold methanol extraction. For POM isotope
 analysis, a known volume of water was filtered through pre-weighed
 pre-combusted (7 h, 500 °C) 0.7 μm glass fiber filters. Upon returning to
 the laboratory, filters were dried for 24 h at 60 °C, weighed and packaged
 in tin capsules for 13C and 15N isotope analysis. Total nitrogen and total
 phosphorus samples were collected in HDPE vials and preserved with H2SO4
 to a pH &lt; 2 and stored at ~ 4 °C until analysis. TN and TP were
 measured using an auto analyzer (LaChat QuikChem 8500, persulfate
 digestions). Leaves of several common plant species were collected from
 shoreline and frozen until processing for isotopic analysis. Leaves were
 sorted into broad functional groups (grasses, shrubs, pine) and dried at
 60 °C for 2 days. A mortar and pestle was used to grind the leaf samples
 before packaging in tin capsules for isotope analysis. Based on a subset
 (10 lakes) of the plant data, we chose to process a grass and pine sample
 to capture the maximum variation in isotopes within the terrestrial
 organic matter entering lakes. To quantify DOC, water
 samples were filtered through pre-combusted glass fiber filters (Whatman
 GF/F, pore size 0.7 μm) into triple-rinsed 20 mL glass vials and preserved
 with HCl to a pH &lt; 2. DOC was measured using a total organic carbon
 analyzer (TOC-V CSN, Shimadzu Scientific Instruments, Japan). To
 characterize DOC quality, we used UV–Vis absorbance, spectrofluorometry
 and spectrophotometry, which reflect several aspects of the molecules
 comprising the light-absorbing and fluorescing DOM pool, respectively. We
 used excitation-emission matrices (EEMs) as a three-dimensional
 representation of fluorescence intensities scanned over a range of
 excitation/emission wavelengths (Coble 1996; Chen et al. 2003). EEMs were
 collected with a JY-Horiba Spex Fluoromax-3 spectrofluorometer (HORIBA,
 Japan) at room temperature using 5 nm excitation and emission slit widths
 and an integration time of 1.0 s. The Aqualog spectrophotometer
 simultaneously collects both fluorescence and absorbance spectra on a
 sample. All fluorescence spectra were collected in signal-to-reference
 (S:R) mode with instrumental bias correction. Instrument-specific
 corrections, Raman area normalization and Milli-Q blank subtraction were
 conducted with Matlab (2009). From the UV–Vis absorbance and EEMs data, we
 calculated two indices of DOC quality: the freshness index (FI) and
 specific UV absorption (SUVA). FI (β:α) is a ratio of emission intensity
 at 380 nm to that of the region between 420 and 435 nm at an excitation of
 310 nm and is reflective of recently produced algal organic matter
 (Parlanti et al. 2000). SUVA is a DOC-normalized index of aromaticity
 calculated as UV absorbance at 254 nm/[(DOC (mg L−1) × path length (0.01
 m)] (Weishaar et al. 2003). FI increases with autochthonous carbon
 production, whereas SUVA increases with allochthonous carbon
 production. All isotope samples were analyzed by the
 University of California, Davis Stable Isotope Facility for 13C and 15N,
 using an elemental analyzer interfaced to a continuous flow isotope ratio
 mass spectrometer. <strong>Fish
 sampling</strong> At each of the 21 lakes, we
 caught fish by angling. Each fish was identified to species, weighed,
 photographed and measured (TL; maximum length). We collected a dorsal
 muscle sample from each individual which was frozen until processing for
 stable isotope analysis. Upon returning to the laboratory, muscle samples
 were freeze dried for 24 h, ground with a mortar and pestle and packaged
 for 13C and 15N analysis. Otoliths were removed, cleaned, dried and stored
 in vials for age determination and growth rate analysis. We calculated
 catch per unit effort (CPUE) as the total catch divided by the number of
 person-hours spent angling at each lake. All applicable institutional
 guidelines for the use of animals were followed and approved by the
 Institutional Animal Care and Use Committee at the University of
 California, San Diego (Protocol #S14140).<br> Fish sample
 processing Fish in temperate regions can be aged by
 examining calcified structures called otoliths, which form annuli—rings
 that correspond to low winter growth. The width of the annuli is an
 indicator of annual growth (Casselman 1990). To determine age and annual
 growth, the sagittal otoliths were mounted on the edge of a microscope
 slide with the core positioned just within the microscope slide’s edge and
 polished to section the otolith. The otolith was then flipped onto the
 transverse cross-section and polished again until the core was exposed in
 the transverse section similar to Taylor and McIlwain (2010). Annuli were
 counted by two independent readers in the absence of information about
 fish size or lake. Ages were in agreement for 84% of the otoliths, and
 never differed by more than 1 year. For otoliths where the age
 determinations disagreed, the two readers examined the otoliths together
 and were able to reach a consensus. The width of each annuli was measured
 using imaging software (Image J).
description: Please see ReadMe files for units.