Diel surface temperature range scales with lake size

Ecological and biogeochemical processes in lakes are strongly dependent upon water temperature. Long-term surface warming of many lakes is unequivocal, but little is known about the comparative magnitude of temperature variation at Diel timescales, due to a lack of appropriately resolved data. Here we quantify the pattern and magnitude of Diel temperature variability of surface waters using high-frequency data from 100 lakes. We show that the near-surface Diel temperature range can be substantial in summer relative to long-term change and, for lakes smaller than 3 km2, increases sharply and predictably with decreasing lake area. Most small lakes included in this study experience average summer Diel ranges in their near-surface temperatures of between 4 and 7°C. Large Diel temperature fluctuations in the majority of lakes undoubtedly influence their structure, function and role in biogeochemical cycles, but the full implications remain largely unexplored

Relationship between the diel range in lake surface water temperature and surface area.

<p>Relationship between the observed (light violet circles) and theoretical (red circles) diel surface temperature range with lake area during summer, with the solid line illustrating the fitted generalised additive model with 95% confidence interval shown by the shaded region; lake surface areas where the diel temperature range changes significantly (P < 0.001) are shown with a red line.</p

Temporal variability in near-surface lake water temperature.

<p>(a) Seasonal variability in the diel temperature range for 96 Northern Hemisphere lakes with 95% confidence intervals (note that not all lakes had data for the whole year). (b) Individually normalized (zero-mean) summer average diel cycle for the lakes that had the highest (red) and lowest (blue) 10% of diel temperature ranges measured. The bold lines represent the mean diel cycle for the 10% considered and the horizontal black line indicates zero. For clarity, we excluded Jekl Bog, which had the highest diel cycle, from this figure. (c) Example of hourly-resolution near-surface lake water temperature variation at Jekl Bog (surface area 2.5 x 10³ m², red), and Sparkling Lake (surface area 6.2 x 10⁵ m², blue), both situated in Wisconsin, USA.</p

First derivatives of the fitted generalised additive model.

<p>The red line indicates those parts of the model fit that are statistically significantly changing and the shaded region shows the 95% confidence intervals.</p

Summary output from the fitted statistical model.

<p>Summary of the model used to describe the influence of surface area (A₀), the percent transmission per metre (I_z), altitude above sea level (h), and latitude (φ), as shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0152466#pone.0152466.e003" target="_blank">Eq 3</a>, on the diel surface temperature range. EDF is the effective degrees of freedom for the spline representing each covariate. Ref. DF is the reference degrees of freedom used in the statistical test of “no effect” for each smooth. F is the test statistic and <i>p</i> the approximate <i>p</i>-value of the test. <i>I</i>_<i>z</i> is the percent transmission per meter.</p

Fitted splines for the Generalised Additive Model.

<p>The y-axis is the additive contribution of the spline to the fitted model over the range of the covariate. The smooth functions are subject to centring constraints and are plotted here on different scales for clarity. The shaded region is an approximate 95% confidence interval on the function; however, it excludes uncertainty in the model's constant term, β₀, hence the narrowness of the interval at the “middle” of the distribution for the smooths of altitude and latitude.</p

Estimated ecological and biogeochemical consequences of the diel surface temperature range.

<p>Potential bias in estimates of CO₂ and O₂ solubility and rates of processes with Q₁₀ values of 2 or 4 for a diel temperature range of 1 (blue) or 7°C (red).</p