Many North American blackwater rivers exhibit low dissolved O₂ (DO) that may be the result of benthic respiration. We examined how tree species affected O₂ demand via the quantity and quality of litter produced. In addition, we compared areal estimates of surface leaf-litter microbial respiration to sediment O₂ demand (SOD) and ecosystem respiration (ER) in stream and swamp reaches of a blackwater river to quantify contributions of surface litter decomposition to O₂ demand. Litter inputs averaged 917 and 678 g m^-2 y^-1 in the swamp and stream, respectively. Tree species differentially affected O₂ demand via the quantity and quality of litter produced. Bald cypress (Taxodium distichum) contributed most litter inputs because of its dominance and because it produced more litter per tree, thereby making greater relative contributions to O₂ demand in the swamp. In the stream, water oak (Quercus nigra) produced litter supporting lower fungal biomass and O₂ uptake rates, but produced more litter than red maple (Acer rubrum). Breakdown rates in the swamp were faster, whereas standing stock decreases were lower than in the stream, indicating greater organic matter retention. Surface litter microbial respiration accounted for 89% of SOD (6.37 g O₂ m^-2 d^-1), and 57 to 89% of ER in the swamp. Our findings suggest that surface litter drives the majority of O₂ demand in some blackwater swamps, and tree species with higher rates of litterfall may make larger contributions to ER. Forested swamps may be hotspots of O₂ demand in blackwater rivers because low water velocities enhance retention.

Disturbance of freshwater ecosystems through cultural eutrophication has resulted in an increased global occurrence of harmful algal blooms (HABs). Ecosystem disrupting algal blooms (EDABs) are a subset of HABs that produce extensive disturbances across entire ecosystems. Prymnesium parvum is an EDAB species that has invaded freshwater systems worldwide, causing massive fish kills and other negative effects. Fish kills frequently occur during HABs and EDABs, but few studies exist of the long-term implications of these fish kills and the resilience and recovery of fish assemblages following kills. We sampled fish near- and offshore over an annual cycle encompassing a P. parvum EDAB in 2 coves (i.e., a bloom site and a reference site) of a southern Great Plains reservoir, Lake Texoma, Oklahoma—Texas (USA). Our objective was to document the extirpation and recovery of a fish assemblage in response to the disturbance of an EDAB event. Prymnesium parvum bloomed in 1 cove from mid-December 2008 until May 2009 and eliminated all fish during this period. Fish toxicity bioassays indicated no substantial differences in susceptibility among fish species to P. parvum toxins. Fish recolonized the bloom site rapidly in May 2009 after the bloom diminished. Fish assemblages were resilient to the P. parvum EDAB, and recovered to previous abundance, richness, and composition within 6 mo. Our results suggest that the reservoir-wide fish meta-assemblage enabled a rapid recovery of local fish assemblages after a spatially heterogeneous EDAB.

Authors of several studies of the spatial distributions of microorganisms have shown strong geographical patterns and stressed the importance of considering the spatial component explicitly when studying assemblage—environment relationships. The processes underlying the patterns are still under debate because it is difficult to separate the unique roles of dispersal limitation and mass effects from spatially structured variation in environment. We analyzed correlations between assemblage dissimilarity and geographical and environmental distances in a large French diatom database, subdivided into regions, years, and different water-quality levels, with multiple regression on distance matrices (MRM) and partial Mantel correlograms. Before we applied MRM, we identified the strongest environmental predictors with the BIO-ENV procedure, which selects the best predictors after testing correlations between distance matrices including every possible set of variables. Environmental control of assemblages was stronger than spatial factors in explaining assemblage patterns, but purely spatial patterns also were significant at the national scale and within some regions. When we averaged environmental and biological data over 3 y, environmental variables accounted for more variability in assemblage structure than relationships estimated with data from a single year. Assemblages in mountainous regions showed particularly strong spatial patterns, perhaps because dispersal barriers hinder the exchange of colonists across sites. The strong spatial structure in the diatom data leads us to encourage researchers to divide ecoregions into smaller areas, especially in mountainous landscapes, when studying assemblage—environment relationships. We also recommend the use of averaged biological and environmental data when developing biotypologies of biotic assemblages for environmental assessment and conservation.

Native freshwater mussels can influence the aquatic N cycle, but the mechanisms and magnitude of this effect are not fully understood. We assessed the effects of Amblema plicata and Lampsilis cardium on N transformations over 72 d in 4 continuous-flow mesocosms, with 2 replicates of 2 treatments (mesocosms with and without mussels), equipped with electronic water-chemistry sensors. We compared sensor data to discrete sample data to assess the effect of additional sensor measurements on the ability to detect mussel-related effects on NO₃^- formation. Analysis of 624 sensor-based data points detected a nearly 6% increase in NO₃^- concentration in overlying water of mesocosms with mussels relative to mesocosms without mussels (p < 0.05), whereas analysis of 36 discrete samples showed no statistical difference in NO₃^- between treatments. Mussels also significantly increased NO₂^- concentrations in the overlying water, but no significant difference in total N was observed. We used the sensor data for phytoplankton-N and NH₄ to infer that digestion times in mussels were 13 ± 6 h. The results suggest that rapid increases in phytoplankton-N levels in the overlying water can lead to decreased lag times between phytoplankton-N and NH₄ maxima. This result indicates that mussels may adjust their digestion rates in response to increased levels of food. The adjustment in digestion time suggests that mussels have a strong response to food availability that can disrupt typical circadian rhythms. Use of sensor data to measure directly and to infer mussel effects on aquatic N transformations at the mesocosm scale could be useful at larger scales in the future.

According to stoichiometric principles, the ratios at which consumers recycle nutrients depend on the elemental compositions of the consumer and its food. However, nutrient assimilation efficiencies and ingestion rates can vary among consumer species and, thus, can affect the rates of consumer-mediated nutrient recycling (CNR). The grazer Theodoxus fluviatilis has high nutrient excretion rates of either P or N, depending on grazer growth limitation, and has a high body N. I examined how a grazer with a high proportion of N in its body tissues can assimilate enough N to maintain that N content despite high N excretion rates by estimating the mass balance for nutrient recycling including nutrient excretion through fecal pellets. I used the snail species Theodoxus fluviatilis and Lymnea peregra fed nutrient-enriched periphyton in a 2-d grazing experiment done in 48 experimental units. I estimated periphyton and grazer nutrient stoichiometry and nutrient excretion rates and ratios in dissolved and fecal-pellet form, and calculated nutrient assimilation efficiencies of the limiting nutrient (N). Theodoxus fluviatilis had higher N excretion rates, lower N assimilation efficiency, and higher ingestion rates than L. peregra. Thus, T. fluviatilis recycled more N by ingesting and processing a larger amount of food per unit time than L. peregra. My study shows that grazers with low nutrient assimilation efficiencies and high nutrient demands can assimilate sufficient nutrients via high ingestion rates. The consequence of this strategy (high ingestion and excretion rates) could be a more rapid nutrient turnover in ecosystems dominated by these grazers.

Data on the functional composition of invertebrates in tropical streams are needed to develop models of ecosystem functioning and to assess anthropogenic effects on ecological condition. We collected macroinvertebrates during dry and wet seasons from pools and riffles in 10 open- and 10 closed-canopy Kenyan highland streams. We classified macroinvertebrates into functional feeding groups (FFGs), which we used to assess effects of riparian condition and season on functional organization. We used cluster analysis of gut contents to classify 86 taxa as collectors, predators, scrapers, or shredders. We classified 23 taxa whose guts were empty or had indistinguishable contents based on literature. In total, we identified 43 predators, 26 collectors, 19 scrapers, and 19 shredders. Total abundance was higher in open-canopy agricultural streams, and species richness was higher in closed-canopy forested streams. Predators and shredders dominated richness and biomass, respectively, in the closed-canopy streams. The shredders, Potamonautes spp. (Brachyura:Potamonautidae) and Tipula spp. (Diptera: Tipulidae), made up >80% of total biomass in most samples containing both. Canopy cover and litter biomass strongly influenced shredder distribution. Seven shredder taxa occurred only in closed-canopy forested streams, and few shredder taxa occurred in areas of low litter input. Collectors dominated abundance at all sites. Richness and biomass of scrapers increased during the dry season, and more shredder taxa were collected during the rainy season. Temperate keys could not be used to assign some tropical invertebrates to FFGs, and examination of gut contents was needed to ascertain their FFGs. The Kenyan highland streams harbor a diverse shredder assemblage that plays an important role in organic matter processing and nutrient cycling.

Predators can induce a suite of evolutionary responses, such as changes in behavior, life-history traits or morphology, from their prey. Our work adds to the growing number of studies of kairomone-mediated aquatic predator—prey interactions that significantly affect prey morphology. Individuals of the amphidromous shrimp Xiphocaris elongata have elongated rostrums below geomorphic barriers where predatory fishes are present and short rostrums in stream reaches above geomorphic barriers where predatory fishes are absent. Our objective was to test whether the elongated rostrum in X. elongata is a phenotypic modification induced by kairomones from predatory fish or alarm cues from conspecifics. We cut the rostrums of juvenile, longrostrum adult, and short-rostrum adult X. elongata and exposed the shrimp to predatory fish fed flakes, predatory fish fed X. elongata, predatory fish fed Atya, the nonpredatory fish S. plumieri, no fish with filtered water, and no fish with stream water. We measured the carapace length and the post-orbital carapace length (to obtain the rostrum length) of every shrimp every 2 wk for a total of 10 times/shrimp. Rostrums of X. elongata exposed to predatory fish fed flakes, predatory fish fed X. elongata, and predatory fish fed Atya shrimp grew longer than X. elongata exposed to the nonpredatory fish, filtered water, or stream water. Rostrums of juveniles grew faster than those of adults independent of treatment. Rostrum growth was not affected by the treatment × phenotype interaction. These results demonstrate that the elongated rostrum in the amphidromous X. elongata is a phenotypic response induced by kairomones from predatory fish.

Nonperennial streams are often excluded from biomonitoring programs because of inadequate knowledge about their biological and hydrological characteristics and variability. The ability to apply bioassessment indices to nonperennial streams would greatly expand the reach of biomonitoring programs. We sampled 12 nonperennial streams (3 of which were minimally stressed) in the San Diego hydrologic region multiple times to assess whether a benthic macroinvertebrate assessment index (the Southern California Index of Biotic Integrity [IBI]) developed for perennial streams could be used in nonperennial streams. We also sampled 3 minimally stressed perennial streams. Continuous water-level loggers and repeated site visits revealed that hydrologic regimes varied considerably among streams. Gradual drying was evident at some streams, and multiple drying/ rewetting events were evident at others. Moreover, streams that were nonperennial in one year were perennial in another. IBI scores from low-stress nonperennial streams were similar to those for low-stress perennial streams, and false indications of impairment (i.e., low IBI scores) were never observed. Furthermore, IBI scores declined as stress increased, suggesting that the IBI responded as expected in nonperennial streams. IBI scores were stable at most sites within and between years, but midsummer declines were observed at high-stress sites. These declines were associated with declines in discharge, fast-water habitat, and increases in sands and fines and macroalgae cover. These findings suggest that an assessment tool developed for perennial streams can be used to assess condition at certain nonperennial streams, and that biomonitoring programs can provide more comprehensive watershed assessments by including nonperennial streams in their surveys.

Lake phytoplankton studies outnumber studies on lake periphyton by an order of magnitude, and most periphyton research has been done in streams. Most benthic algal taxa found in lakes also can be found in lotic systems, but assemblages and taxa differ in a number of ways. The ecological characteristics of some lake benthic algae reflect habitat coupling. Littoral zones (benthic areas above the light compensation depth) are areas of high productivity and biodiversity. The proportion of benthic and planktonic primary production (autotrophic structure) is a key ecosystem property. The distribution of lake benthic algae is markedly influenced by the depth gradient and substratum, and assemblages change with depth from epilithic and epiphytic rheobionts to epipsammic and epipelic limnobionts. At shallow depths, periphyton must cope with the effects of high radiation, water-level fluctuations, wave action (e.g., desiccation, ultraviolet radiation exposure, shear stress), and seasonal shifts in temperature. This situation selects for widely distributed rheophilic species. In contrast, the deeper littoral zone is stable and hosts a distinct subset of lentic periphyton. However, species experience low light intensity, which becomes increasingly severe with increasing depth, and are often threatened by eutrophication-driven increased shading by phytoplankton. Besides change across depths and substrata, adaptations to disturbance levels, competition, microdistribution of phenological stages, and physiomorphological regulation generate multiple and complex spatial patterns at different scales. Lake shores are the focus of human activities with significant consequences for periphyton. In this review, we introduce a series of 15 papers on the topic and suggest directions for future research. Overall, this special series illustrates that, despite the many important ecosystem services provided by lake benthic algae, the topic is understudied.

Primary production in lakes occurs in both planktonic (water column) and benthic (bottom) habitats. How whole-lake primary production is distributed between these 2 habitats—referred to as autotrophic structure—is a key ecosystem property. Empirical research examining the balance between benthic and planktonic primary production in lakes is scarce, and how autotrophic structure changes across depth, nutrient, water clarity, and biological invasion gradients is unclear. Therefore, we are ill equipped to anticipate ecosystem-level responses to environmental change. We assessed the magnitude of offshore planktonic, nearshore planktonic, and nearshore benthic gross primary production (GPP) along a gradient of nutrients, water clarity, and Cladophora biomass in Green Bay, Lake Michigan, USA, during summer 2010, 2011, and 2012. Benthic and planktonic GPP varied strongly along the trophic gradient. Planktonic GPP increased with nutrients status, whereas benthic GPP decreased. From shore to 10 m depth, autotrophic structure shifted from planktonic dominance near the mouth of the Fox River (95% planktonic) to a mix of benthic and planktonic GPP 35 km from the mouth of the Fox (∼40% benthic). The steep bathymetry at more-distant sites reduced the relative importance of benthic GPP at the whole-ecosystem level. Our work highlights the dual-pathway (i.e., benthic and planktonic) nature of lentic food webs from the perspective of GPP, and shows how both trophic status and bathymetry affect autotrophic structure.

Lake Tovel is an oligotrophic, meromictic, mountain lake of the Dolomites that undergoes marked seasonal water-level fluctuations (WLFs). We used neo- and paleolimnological data collected since 1999 to test the utility of algal and cyanobacterial pigments and diatom and chironomid biodiversity as proxies for WLF and to highlight the contribution of benthic algae to the sediment record. We found that detailed knowledge of presentday spatiotemporal patterns of WLF is essential for a complete and correct interpretation of paleolimnological data. Scytonemin, a cyanobacteria-specific photoprotective pigment, was produced by epilithic cyanobacteria in the depth zone affected by WLF and should be considered a proxy for the extent of WLF. The phytobenthos was as important as phytoplankton as a source of sedimentary pigments. We used information gained on the detailed distribution of diatoms at different depths and on different substrata in the 2 basins to show the probable location in the lake from which taxa in sediment cores originated to aid in interpretation of the sediment archive (including identification of periods of active hydrology). We sampled present-day chironomids in springs and streams feeding the lake and along a depth transect in the main basin. The taxa found were all rheophilic, crenophilous, or typical of the littoral zone of oligotrophic lakes. We interpreted sections of the cores containing large numbers of Orthocladiinae and Diamesinae head capsules as indicative of periods of active hydrology (including sudden and marked WLF) because the littoral taxa found in the cores must have been derived from the littoral zone by sediment focusing or slumps. Thus, we identified useful proxies of WLF in mountain, carbonate lakes.

Epipelic microphototrophs perform a range of ecosystem functions including biostabilization of sediments, regulation of benthic—limnetic nutrient cycling, and primary production. This review summarizes the most recent research of lake/pond phototrophic epipelic taxa and highlights the importance of an approach to identifying these algae that combines classic and modern methods, particularly metagenomics and sequencing. The contribution of true epipelic chlorophyll a concentration to total sediment chlorophyll a production can reach 30%, particularly at sites of intermediate depth. Autochthonous epipelic assemblages (diatoms, cyanobacteria, desmids, and euglenophytes) are highly diverse, but overlooked. The result has been recent erection of new genera and species. Species distribution seems to follow ecological gradients (pH, conductivity, trophy) and can be used in biomonitoring.

Periphyton and phytoplankton contribute to the base of lake food webs, and both groups of microalgae are influenced by resources and physical forcing. Spatial variation in light availability interacts with the physical dynamics of the water column to create predictable depth gradients in resources and disturbance that may differentially affect periphyton vs phytoplankton. We characterized the depth distribution of chlorophyll and productivity of periphyton on sediments (epipelon) and phytoplankton in the euphotic zones of 13 oligomesotrophic lakes that span a large size gradient (0.017–32,600 km²). Epipelic chlorophyll usually increased with depth in the epilimnion. Light was the primary driver of the consistent within-lake patterns in periphyton productivity across this lake-size gradient. In 5 lakes, epipelic periphyton exhibited a unimodal distribution of productivity with depth in the photic zone, but no evidence of photoinhibition was found for periphyton. Rather, patterns in sediment N and P and observed changes in biofilm structure were consistent with determination of epipelic biomass by disturbance at depths ≤1 m in the smaller lakes and by light limitation at depths >1 m. Further quantification of the effects of disturbance on epipelon is needed. Nonetheless, our data demonstrate that the perceived high spatial variability in periphyton biomass and productivity is not an impediment to development of robust models of whole-lake primary production that include both phytoplankton and periphyton.

Lake benthic cyanobacteria and algae are distributed along a depth gradient. At the extremes of the gradient, irradiance is a selecting factor because of excess (UV exposure in shallow waters) or extreme reduction (light limitation at the bottom of the euphotic zone). We tested whether, how, and to what extent epilithic cyanobacteria assemblages change with depth in a carbonate meromictic lake (Lake Tovel, southeastern Alps) that undergoes marked water-level fluctuations. Fixed stations were placed along a transect at 2- to 3-m depth intervals from 0 to 24 m and were sampled by SCUBA divers. Three depth-distribution zones were identified based on the composition of cyanobacterial assemblages and primary benthic algal pigments: shallow, middepth, and deep. The autecological traits of the cyanobacteria indicators identified by indicator value (IndVal) analysis suggested that the shallow, mid-depth, and deep zones were disturbed by water-level fluctuations, highly stable with favorable growth conditions, and severely light-limited, respectively. The shallow zone was colonized mainly by pseudaerial cyanobacteria and by UV-resisting phenoecodemes with yellow—brown sheaths (scytonemin). Shannon—Wiener diversity was highest in the shallow zone, probably because taxa characteristic of periodically inundated habitats mixed with lacustrine taxa. The most unique morphospecies were in the lower part of the mid-depth and in the deep zone. These species frequently had colorless sheaths or pink—red—violet cell contents. We found clear patterns in the depth distribution of benthic cyanobacteria and algal pigments that indicated adaptation to the principal evolutionary pressures at the extremes of the depth gradient: water-level fluctuation and light attenuation. These features have implications for quality assessments, biodiversity inventories, and identification of areas affected by water-level fluctuations.

The deep littoral zone hosts a unique but understudied subset of lake periphyton. We investigated epilithic cyanoprokaryotes in a carbonate meromictic lake (Lake Tovel, southeastern Alps) to obtain information on 2 poorly known, deep-dwelling coccoid species (one new to science) and to analyze their distribution in light of the current understanding of the depth distribution of lake periphyton. Cyanoprokaryotes in Lake Tovel have distinct depth distributions among 3 zones: shallow (0–4 m), mid-depth (6–12 m), and deep (15–24 m). The mid-depth and deep zones are characterized by high stability with favorable growth conditions and by severe light limitation, respectively. Both zones have unique taxa that frequently possess colorless sheaths or pink—red— violet cell contents. The 2 most poorly known cyanoprokaryotes found in Lake Tovel (both pink—red—violet) were a species of Geitleribactron new to science (Geitleribactron purpureum sp. nov. Cantonati et Komárek) and the rare Chlorogloea purpurea. We described and documented, respectively, these 2 species by means of light microscopy, careful comparison with the most-similar morphospecies, transmission electron microscopy (TEM), and ecological characteristics. We are the first to describe the TEM ultrastructure of a Chlorogloea species and to characterize the autecology and distribution of C. purpurea, which, in spite of its very specific habitat requirements (deep waters of carbonate oligotrophic mountain lakes), has been reported in several studies from disparate habitats and geographic locations.