Peter A. Staehr

Reconciling the temperature dependence of respiration across timescales and ecosystem types

Ecosystem respiration is the biotic conversion of organic carbon to carbon dioxide by all of the organisms in an ecosystem, including both consumers and primary producers. Respiration exhibits an exponential temperature dependence at the subcellular and individual levels, but at the ecosystem level respiration can be modified by many variables including community abundance and biomass, which vary substantially among ecosystems. Despite its importance for predicting the responses of the biosphere to climate change, it is as yet unknown whether the temperature dependence of ecosystem respiration varies systematically between aquatic and terrestrial environments. Here we use the largest database of respiratory measurements yet compiled to show that the sensitivity of ecosystem respiration to seasonal changes in temperature is remarkably similar for diverse environments encompassing lakes, rivers, estuaries, the open ocean and forested and non-forested terrestrial ecosystems, with an average activation energy similar to that of the respiratory complex (approximately 0.65 electronvolts (eV)). By contrast, annual ecosystem respiration shows a substantially greater temperature dependence across aquatic (approximately 0.65 eV) versus terrestrial ecosystems (approximately 0.32 eV) that span broad geographic gradients in temperature. Using a model derived from metabolic theory, these findings can be reconciled by similarities in the biochemical kinetics of metabolism at the subcellular level, and fundamental differences in the importance of other variables besides temperature—such as primary productivity and allochthonous carbon inputs—on the structure of aquatic and terrestrial biota at the community level.

Decreasing resilience of kelp beds along a latitudinal temperature gradient: potential implications for a warmer future

Successful mitigation of negative effects of global warming will depend on understanding the link between physiological and ecological responses of key species. We show that while metabolic adjustment may assist Australasian kelp beds to persist and maintain abundance in warmer waters, it also reduces the physiological responsiveness of kelps to perturbation, and suppresses canopy recovery from disturbances by reducing the ecological performance of kelp recruits. This provides a warning not to rely solely on inventories of distribution and abundance to evaluate ecosystem function. The erosion of resilience is mediated by a shift in adult-juvenile interactions from competitive under cool to facilitative under warm conditions, supporting the prediction that positive interactions may become increasingly important in a warmer future. Kelp beds may remain intact but with a lower threshold for where additional impacts (e.g., extreme storms or reduced water quality) will lead to persistent loss of habitat and ecological function.

PHYSIOLOGICAL RESPONSES OF ECKLONIA RADIATA (LAMINARIALES) TO A LATITUDINAL GRADIENT IN OCEAN TEMPERATURE 1

We tested the ability of sporophytes of a small kelp, Ecklonia radiata (C. Agardh) J. Agardh, to adjust their photosynthesis, respiration, and cellular processes to increasingly warm ocean climates along a latitudinal gradient in ocean temperature (∼4°C). Tissue concentrations of pigment and nutrients decreased with increasing ocean temperature. Concurrently, a number of gradual changes in the metabolic balance of E. radiata took place along the latitudinal gradient. Warm‐acclimatized kelps had 50% lower photosynthetic rates and 90% lower respiration rates at the optimum temperature than did cool‐acclimatized kelps. A reduction in temperature sensitivity was also observed as a reduction in Q10‐values from cool‐ to warm‐acclimatized kelps for gross photosynthesis (Q10: 3.35 to 1.45) and respiration (Q10: 3.82 to 1.65). Respiration rates were more sensitive to increasing experimental temperatures (10% higher Q10‐values) than photosynthesis and had a higher optimum temperature, irrespective of sampling location. To maintain a positive carbon balance, E. radiata increased the critical light demand (Ec) exponentially with increasing experimental temperature. The temperature dependency of Ec was, however, weakened with increasing ocean temperature, such that the critical light demand was relaxed in kelp acclimated to higher ocean temperatures. Nevertheless, calculations of critical depth limits suggested that direct effects of future temperature increases are unlikely to be as strong as effects of reduced water clarity, another globally increasing problem in coastal areas.

Epibiota communities of the introduced and indigenous macroalgal relatives Sargassum muticum and Halidrys siliquosa in Limfjorden (Denmark)

Sargassum muticum (Phaeophyceae, Fucales) has recently been introduced to Limfjorden (Denmark) where its closest relative is the indigenous Halidrys siliquosa. Previous studies have demonstrated large quantitative (canopy biomass) and qualitative (canopy persistence) differences in the habitat available to epibiota within the canopies of these two macroalgae. We therefore hypothesised that these algae would support different epibiota communities and tested this by sampling the epibiota of S. muticum and H. siliquosa on seven occasions throughout 1997 by enclosing entire thalli in mesh bags. We found 53 epibiota taxa and, with only one exception, they were all recorded on both host species. Species richness and abundance of epibiota exhibited clear seasonal variation on both host species, although epibiota biomass was seasonally constant on H. siliquosa but not on S. muticum. These patterns were consistent with the different life histories of the host species. There was a weakly negative correlation between thallus size and epibiota biomass for both host species. When taking species-specific seasonal variation in thallus size into consideration, S. muticum and H. siliquosa were found to support significantly different epibiota biomasses. Multivariate analyses showed that epibiota community structure was different, although highly overlapping, between the two species, whereas there was an almost parallel temporal development in epibiota community structure. We conclude that it is unlikely that the introduction of S. muticum to Limfjorden has caused major changes in local epibiota community structure. However, the standing stock of epibiota is likely to have increased.

Temperature acclimation of growth, photosynthesis and respiration in two mesophilic phytoplankton species

P.A. Staehr and M.J. Birkeland. 2006. Temperature acclimation of growth, photosynthesis and respiration in two mesophilic phytoplankton species. Phycologia 45: 648–656. DOI: 10.2216/06-04.1 Temperature acclimation in two mesophilic microalgae, Microcystis aeruginosa (Cyanobacteriales) and Scenedesmus acutus (Chlorococcales), was studied by measuring growth rate, photosynthesis, respiration, cell size, cellular pigment content and Chl a-specific light absorption. Phytoplankton were grown as nutrient-replete semicontinuous cultures for 2 weeks at 5, 15 and 25°C, during which growth rate was determined from changes in Chl a. Gross photosynthesis (GP) was measured as 14C assimilation at saturating light and respiration (R) was measured as O2 uptake along a temperature gradient from 0 to 40°C. Net photosynthesis (NP) was determined as the difference between GP and R. For both species, acclimation to increasing growth temperatures resulted in increasing growth rate, cellular pigment content and decreasing cell size and Chl a-specific light absorption. Scenedesmus acutus and M. aeruginosa showed the same overall pattern of metabolic acclimation to increasing temperatures: (1) overall higher GP and NP but lower R; (2) increasing optimum temperatures for GP, NP and R and (3) higher metabolic rates at supraoptimal temperatures. Microcystis aeruginosa showed several warm-loving traits. It was more sensitive to increasing temperatures (higher Q10 values), had higher metabolic rates and optimum temperatures and performed better at high incubation temperatures than S. acutus did. This study shows that phytoplankton have a considerable and rapid ability to adjust cellular physiology, metabolism and growth to relatively large changes in growth temperature. This suggests a significant ability to acclimate to increasing temperatures associated with forthcoming climate changes.

Net Heterotrophy in Small Danish Lakes: A Widespread Feature Over Gradients in Trophic Status and Land Cover

Nineteen small lakes located in open landscapes or deciduous forests in nutrient-rich calcareous moraines in North Zealand, Denmark, were all net heterotrophic having negative net ecosystem production and predominant CO2 supersaturation and O2 undersaturation of lake waters. Forest lakes were poorer in nutrients, phytoplankton, and primary production, but richer in dissolved organic matter and CO2 than open lakes with more light available. The modeled annual balance between gross primary production and community respiration (GPP/RCOM) averaged 0.60 in forest lakes and 0.76 in open lakes and the ratio increased significantly with phosphorus concentration and phytoplankton biomass but decreased with colored dissolved organic matter. The negative daily rates of ecosystem production resembled estimates of oxygen uptake from the atmosphere to the lakes, whereas estimates of CO2 emission were 7.2-fold higher. Although CO2-rich groundwater and anaerobic respiration support greater molar release of CO2 than uptake of O2, we suggest CO2 emission is overestimated. Possible explanations include CO2 enrichment of the air film above small wind-sheltered lakes. The observed metabolism and gas exchange show that exogenous organic matter is an important supplementary energy source to community respiration in these small lakes and that forest lakes, in addition, experience pronounced light attenuation from trees and dissolved colored organic matter constraining primary production.

Parameterization of the chlorophyll a-specific in vivo light absorption coefficient covering estuarine, coastal and oceanic waters

We evaluated models predicting the spectral chlorophyll-a (Chl a)-specific absorption coefficient (a* ph (λ)) from Chl a concentration [Chl a] on the basis of 465 phytoplankton absorption spectra collected in estuarine, coastal and oceanic waters. A power model on ln-transformed data provided the best model fit compared to a power model on non-transformed data previously applied to parameterize the relationship between a* ph (λ) and [Chl a]. The variation in a* ph (λ) was parameterized over four orders of magnitude in [Chl a] (0.01-100 mg Chl a m−3) producing a 13-fold range in a* ph (0.19 to 0.015 m2 mg−1 Chl a) at 440 nm, the peak absorption of Chl a in the blue part of the spectrum. The variations in the modelled a* ph spectra were within realistic predictions of a* ph (λ) and the model satisfactorily reproduced the spectral flattening with increasing [Chl a]. The parameterization of a* ph (λ) confirmed the indirect dependency of a* ph (λ) on [Chl a] through co-variations between [Chl a] with pigment packaging and pigment composition. Although pigment packaging determined the spectral flattening, analysis of absorption ratios revealed a systematic change in pigment composition with profound influence on the variability of a* ph in the 440 to 495 nm region. Modelled spectra deviated by approximately 20% from the measured spectra on average and model accuracy was independent of [Chl a]. Although the model cannot fully replace spectral measurements of phytoplankton absorption, it does permit realistic reconstructions of a* ph (λ) from simple measurements of [Chl a] sampled in estuarine, coastal and oceanic waters.

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.

Hydrological Conditions Control P Loading and Aquatic Metabolism in an Oligotrophic, Subtropical Estuary

Using high-resolution measures of aquatic ecosystem metabolism and water quality, we investigated the importance of hydrological inputs of phosphorus (P) on ecosystem dynamics in the oligotrophic, P-limited coastal Everglades. Due to low nutrient status and relatively large inputs of terrestrial organic matter, we hypothesized that the ponds in this region would be strongly net heterotrophic and that pond gross primary production (GPP) and respiration (R) would be the greatest during the “dry,” euhaline estuarine season that coincides with increased P availability. Results indicated that metabolism rates were consistently associated with elevated upstream total phosphorus and salinity concentrations. Pulses in aquatic metabolism rates were coupled to the timing of P supply from groundwater upwelling as well as a potential suite of hydrobiogeochemical interactions. We provide evidence that freshwater discharge has observable impacts on aquatic ecosystem function in the oligotrophic estuaries of the Florida Everglades by controlling the availability of P to the ecosystem. Future water management decisions in South Florida must include the impact of changes in water delivery on downstream estuaries.

Surface microlayers on temperate lowland lakes

At the air–water interface material, organisms accumulate and form a thin layer of organic and inorganic material called the surface microlayer (SML). In order to investigate the development, composition, and metabolism of SML on lakes, samples were collected using a screen sampler along with subsurface water (SSW) in an eutrophic and a mesotrophic lake from April to September 2007. Wind, solar irradiance, and lake temperature were followed continuously. Samples were analyzed for organic and inorganic compounds as well as for photosynthesis and respiration. Most compounds were enriched in the SML relative to the SSW. Enrichment was small, however, probably because sampling was performed on nonslick areas. Most compounds correlated closely between the SML and the SSW, confirming the hypothesis that most SML material originates from the bulk water. Correlations were strongest in the eutrophic lake, probably because external sources had a greater effect on SML concentrations in the mesotrophic lake. Enrichment of compounds and metabolic rates in the SML had similar seasonality and dependency of climatic conditions in the two lakes, suggesting common regulating mechanisms of enrichment and production. Enrichment factors of several compounds were higher at low bulk water concentrations, suggesting that atmospheric deposition then contributed relatively more to concentrations in the SML. Increasing temperature significantly decreased SML enrichment of TOC (total organic carbon), related to changes in TOC composition and higher heterotrophic activity, while wind and solar irradiance had no pronounced enrichment effect on any compound. Net photosynthesis was significantly lower in the SML, experiencing photoinhibition in one-third of the samples. In contrast, respiration was much elevated in the SML. Nonetheless, respiration in the SML never contributed by more than 0.3% of water column respiration, but the combination of enhanced degradation rates of organic carbon in the SML and strong interaction with water below suggests that the SML, nonetheless, may play an important role in degradation of refractory organic carbon. Combining these results, we found that the SML of nonslicked areas on lakes are enriched in organic and inorganic pools and constitute a strong heterotrophic environment, albeit of minor importance for whole lake pelagic metabolism.