The Role of Abiotic and Biotic Processes in Regulating Benthic Ecosystem Function along a Productivity Gradient
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The Role of Abiotic and Biotic Processes in Regulating Benthic Ecosystem Function along a Productivity Gradient


The benthic zone of lakes can be a hotspot for lake energy flows and nutrient cycling. Indeed, the notion of benthic “ooze” and its role in the recycling of nutrients forms a conceptual cornerstone of ecology (Lindeman 1942). Benthic habitats can vary from soft, muddy sediments rich in organic matter and nutrients to bare rock covered by thin biofilms. These habitats form the linkage between lake and watershed, and support a range of ecosystem processes—from exchange of groundwater nutrients, internal nutrient cycling, mineralization of organic matter, heterotrophic respiration, and benthic algal production—that each vary in importance between different waterbodies. Clear Lake, CA, and Lake Tahoe represent two systems that span a gradient in trophic level, depth, and underlying geology. The former is shallow, and extremely productive, while the latter is deep and has nutrient concentrations near detection limit. Both, however, are large lakes and are extremely important water resources in California. We set out to characterize two very different, but nevertheless important, benthic processes that are adversely affecting water quality in each lake.Benthic algae, or periphyton, blooms may be increasing in oligotrophic, clear water lakes globally in response to increases in water temperature and nutrient loads, often associated with climate change. In many regions, climate change is altering the duration of snow cover, the frequency and severity of drought, and storm intensity, all of which have the potential to affect periphyton growth rates. Lake Tahoe, located in the Sierra Nevada Mountains of California and Nevada, has anecdotal reports that the frequency and severity of nearshore periphyton blooms have increased in recent years. Such blooms decrease both water quality and the aesthetic value of nearshore areas. While recent studies have documented changes in periphyton biomass, it is difficult for in situ studies to disentangle the individual and interactive effects of temperature and nutrients on periphyton growth rates. Consequently, we used laboratory experiments to examine 1) seasonal variability in periphyton biomass and metabolism, 2) the role of temperature, nutrients, and their interactive effects on determining periphyton metabolic rates, and 3) seasonal variability of the aforementioned temperature and nutrient effects. By measuring changes in dissolved oxygen in sealed incubation chambers containing rocks collected from the nearshore, we quantified rates of gross primary production (GPP), ecosystem respiration (ER), and net ecosystem production (NEP). Our experiments had two nutrient treatments (ambient and augmented with nitrogen and phosphorus) and four temperature treatments (ambient, +3, +6, and +9 °C above ambient) to simulate warming during different seasons, and nutrient loading associated with increased runoff. We found that temperature drove the majority of variability in periphyton metabolic rates and increased GPP, ER, and NEP by 4-7% per 1°C of warming. The effect of warming varied seasonally with larger effects in colder months. Warming also stimulated ER more so than GPP, with decreasing trends of NEP with warming during summer months. Nutrient additions had variable effects on rates of GPP, ER and NEP, but increased ER rates by 6-7% at maximum nutrient concentrations observed in the field. However, in contrast to our expectations, we found little evidence for interactive effects between temperature and nutrients for GPP and NEP, and a small, negative, inhibitory interaction for ER. In all, these experiments provide evidence that warming can increase periphyton metabolic rates in oligotrophic systems even in the absence of increased nutrient loads in cases where periphyton are not nutrient limited. These effects on littoral zones of lakes will have important consequences for ecosystem structure and function, and management efforts to curb nutrients will be crucial for limiting periphyton blooms under climate change. Eutrophication continues to be a problem in aquatic systems globally, resulting from excessive nutrients such as phosphorus and nitrogen. Nutrient sources can be external (i.e. runoff) or from within the lake (internal loading). In the case of phosphorus, internal loading can occur several ways: 1) chemically reduced conditions at the sediment-water interface of lake bottoms (usually induced by anoxic conditions) facilitate the reductive dissolution of phosphorus from the sediments to the overlying water, 2) mineralization of organic matter, and 3) physical resuspension of shallow sediments, and 4) elevated pH (commonly the result of intensive algal productivity) that results in ion exchange in sediments. Efforts to restore Clear Lake, a hypereutrophic lake located in Lake County, CA, have largely focused on external nutrient inputs as the cause of excessive phosphorus concentrations. This study provided the first direct measure of internal phosphorus fluxes from the nutrient-rich lake-bottom sediments to the overlying water column using incubations of intact sediment cores. Similar to other studies, soluble reactive phosphorus (SRP) flux rates from sediments ranged from 8.8 to 26.7 mg SRP m-2 d-1 in anoxic treatments and from -4.2 to 0.9 mg SRP m-2 d-1 in oxygenated treatments. We found little seasonal variation in anoxic SRP flux rates between winter and summer incubations despite large temperature differences between these two seasons. We hypothesize that early summer drawdown of redox-sensitive sediment P pools can potentially limit sediment phosphorus flux rates later in the season despite the positive effects of warmer summer temperatures on SRP flux. Thus, characterizing phenology of sediment P pools is important for estimating annual internal phosphorus loads in lakes. Large internal fluxes of P in this system were largely ignored during establishment of nutrient TMDLs for the watershed. This study highlights the importance of measuring internal fluxes of P and its seasonality when setting nutrient TMDLs or designing restoration strategies for a watershed.

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