Metabolic compensation constrains the temperature dependence of gross primary production

Abstract Gross primary production (GPP) is the largest flux in the carbon cycle, yet its response to global warming is highly uncertain. The temperature dependence of GPP is directly linked to photosynthetic physiology, but the response of GPP to warming over longer timescales could also be shaped by ecological and evolutionary processes that drive variation in community structure and functional trait distributions. Here, we show that selection on photosynthetic traits within and across taxa dampens the effects of temperature on GPP across a catchment of geothermally heated streams. Autotrophs from cold streams had higher photosynthetic rates and after accounting for differences in biomass among sites, biomass‐specific GPP was independent of temperature in spite of a 20 °C thermal gradient. Our results suggest that temperature compensation of photosynthetic rates constrains the long‐term temperature dependence of GPP, and highlights the importance of considering physiological, ecological and evolutionary mechanisms when predicting how ecosystem‐level processes respond to warming.


Measuring the organism-level metabolic thermal response
We sampled 13 of the most abundant macroscopic cyanobacteria, filamentous eukaryotic algae, and bryophyte taxa from 8 streams spanning the catchment's full thermal gradient to characterise their metabolic thermal responses using an O 2 electrode system. Multiple taxa were sampled from four streams where more than one taxon was at high density (Table S6). Because we sampled macroscopic algae -e.g. crops of filamentous algae or bryophyte fronds -measurements of metabolic rate are assumed to be at the level of the focal organism. We acknowledge that commensal microbes (e.g. protists and bacteria) are likely to be associated with these samples, but we assume that these organisms contribute a tiny fraction of the total biomass relative to the focal organism. Given the sensitivity of the O 2 electrode, these commensal organisms likely make a negligible contribution to the measurements of metabolism.
Rocks dominated by a focal alga were brought back to the laboratory and maintained in water from their natal stream over the course of the metabolic measurements.

Measuring in situ rates of ecosystem-level gross primary production
Ecosystem metabolism was calculated from measurements of dissolved oxygen over time in each stream using the single station method (Odum 1956). Sensors were deployed in all streams and at multiple sites within a stream where temperature gradients existed within streams due to differential geothermal warming. Dissolved oxygen concentration and temperature were monitored at 1-minute intervals using miniDOT optical dissolved oxygen loggers (PME Inc) (Fig. S3 & Fig. S5). Light  (Table S4).
The change in O 2 concentration at a single station between two subsequent measurements (∆DO) can be approximated as: with [O 2 ] t the concentration of oxygen (mg L -1 ) at time t and can be modelled using a framework based on the Odum's O 2 change technique (Odum 1956): where △ jk is the composite of volumetric gross primary productivity, noo (g m -3 min -1 ), minus volumetric ecosystem respiration, *p (g m -3 min -1 ) and n is the net exchange of oxygen with the atmosphere (g O 2 m -3  (Table S3).
In 2016, we also measured autotrophic biomass density (g Chl a m -2 ) across the catchment by taking measurements of chlorophyll a. Autotrophic biomass was estimated by removing all organic material from a 30 cm 2 template on 3 randomly chosen rocks from each stream. Biofilm and plant material was removed from within the sample area using forceps and a stiff bristled brush, rinsed with distilled water and the slurry decanted into a Falcon tube. Chlorophyll was then extracted and quantified using the protocol detailed above. The total autotrophic biomass, y z , of each stream reach was estimated by multiplying average autotrophic biomass density by the total reach area, which was estimated from the mean width and the upstream distance the were calculated by dividing areal rates of GPP by the total autotrophic biomass, y z , in the upstream reach.

Comparison of measured and modelled reaeration rates
To assess the robustness of our modelled values of reaeration, we compared measurements of the reaeration rate made in nearby streams in Iceland with comparable physical characteristics using propane additions (from Demars et al.

2011)
, to values estimated using the surface renewal model (eq. 14, main text). In Demars et al. (2011), the reaeration rate was measured using a tracer study, where propane was bubbled continuously across the width of the stream at an upstream station. Water samples were taken at a downstream station and analysed by gas chromatography back in the laboratory (see for a more detailed description of the methods). The change in propane concentration the over the reach and the travel time were used to estimate the reaaeration rate, s (min -1 ).
We compared the measured values of reaeration, s (min -1 ), from Demars et al.
(2011) to estimated values of s derived Eq. 14 (main text) and measurements of velocity, depth and temperature for those streams. We found a strong correlation between modelled and measured values of s with 95% confidence intervals on the slope that included unity (slope = 1.13, 95% CI: 0.76 -1.50) and an R 2 = 0.61 (Fig.   S8). In addition, we examined potential biases by plotting the residuals of the ln-ln plot of modelled vs measured reaeration against stream temperature (see Fig. S9).
This analysis demonstrates that the model residuals do not vary systematically with stream temperature. Consequently, we are confident that estimates of reaeration derived from the surface renewal model are robust for the streams included in our survey. Figure S8. Comparison of modelled and measured rates of reaeration. Rates of measured reaeration using a propane tracer study are positively correlated with those derived using the surface renewal model (eq. 14; main text) with slope that was statistically indistinguishable from unity.