STRUCTURE AND FUNCTION OF BENTHIC ALGAL COMMUNITIES IN AN EXTREMELY ACID RIVER1

The composition of algal species and pigments and the structural and functional characteristics of the algal community were investigated in an acid stream of southwestern Spain, the Río Tinto. The algal community had low diversity and showed few seasonal differences. It was mainly made up of Klebsormidium flaccidum Kütz. (Silva, Mattox & Blackwell) that produced long greenish or purplish filaments, Pinnularia acoricola Hust. (producing brown patches) and Euglena mutabilis Schmitz. The algal filaments made up a consistent biofilm that also included fungal hyphae, iron bacterial sheaths, diatoms, and mineral particles. HPLC analyses on Río Tinto samples showed that undegraded chl accounted for 67% of the total chl in the filamentous patches but were a minority in the brown patch (2.6%). The brown patch had a concentration of carotenoids eight times lower than that observed in the green patch. When chl concentrations were weighted for the proportion of the different patches on the streambed, undegraded chl a accounted for 89.2 mg chl a·m ^{? 2} of stream surface area (5.4 g C·m ^{? 2}). This high algal biomass was supported by relatively high nutrient concentrations and by a high phosphatase activity (V_max = 137.7 nmol methylumbelliferyl substrate·cm ^{? 2}·h ^{? 1} 1 Received 15 July 2002. Accepted 17 February 2003. , K_m = 0.0045 μM). The remarkable algal biomass in Río Tinto potentially contributed to the bacterial–fungal community and to the macroinvertebrate community and emphasizes the role that the algae may have in the organic matter cycling and energy flow in extreme systems dominated by heterotrophic microorganisms.     

A comparison of HPLC pigment analyses and biovolume estimates of phytoplankton groups in an oligotrophic lake

Assessment of the contribution of distinct algal groups to phytoplanktonbiomass in oligotrophic lakes by marker pigments is comparedwith assessment by cell-counting biovolume estimates. Seasonalsamples from an oligotrophic alpine lake (Redon, Pyrenees) mostlyincluded species of chrysophytes, dinoflagellates, cryptophytesand chlorophytes. The chlorophyl a (Chl a) corresponding toeach algal group was estimated using HPLC pigment analyses andthe CHEMTAX program. Chl a estimates and biovolume showed asignificant correlation for all the groups during the ice-freeseason except for chlorophytes. However, some of the samplesfrom the initial phase of the ice cover presented a clear departurefrom the relationship during the ice-free period in most groups.On the other hand, the ratios between a specific marker pigmentand the biovolume of the marked algal group were significantlyconstant within the photic zone (>1% surface irradiance)for most of the pigments and groups, including chlorophytes.Nevertheless, the ratios increased and showed a large variabilityfor samples below the photic depth or below the ice cover. Theviolaxanthin-chrysophyte biovolume ratio presented an opposedtendency to other pigment-biovolume ratios, which increasedin inverse proportion to the depth of the sample. The resultsare discussed in terms of methodological limitations, acclimationresponses and species composition.     

Acceleration of cyanobacterial dominance in north temperate‐subarctic lakes during the Anthropocene

Increases in atmospheric temperature and nutrients from land are thought to be promoting the expansion of harmful cyanobacteria in lakes worldwide, yet to date there has been no quantitative synthesis of long‐term trends. To test whether cyanobacteria have increased in abundance over the past ~ 200 years and evaluate the relative influence of potential causal mechanisms, we synthesised 108 highly resolved sedimentary time series and 18 decadal‐scale monitoring records from north temperate‐subarctic lakes. We demonstrate that: (1) cyanobacteria have increased significantly since c. 1800 ce , (2) they have increased disproportionately relative to other phytoplankton, and (3) cyanobacteria increased more rapidly post c. 1945 ce . Variation among lakes in the rates of increase was explained best by nutrient concentration (phosphorus and nitrogen), and temperature was of secondary importance. Although cyanobacterial biomass has declined in some managed lakes with reduced nutrient influx, the larger spatio‐temporal scale of sedimentary records show continued increases in cyanobacteria throughout the north temperate‐subarctic regions.     

Multiple glacial refugia and contemporary dispersal shape the genetic structure of an endemic amphibian from the Pyrenees

Historical factors (colonization scenarios, demographic oscillations) and contemporary processes (population connectivity, current population size) largely contribute to shaping species’ present‐day genetic diversity and structure. In this study, we use a combination of mitochondrial and nuclear DNA markers to understand the role of Quaternary climatic oscillations and present‐day gene flow dynamics in determining the genetic diversity and structure of the newt Calotriton asper (Al. Dugès, 1852), endemic to the Pyrenees. Mitochondrial DNA did not show a clear phylogeographic pattern and presented low levels of variation. In contrast, microsatellites revealed five major genetic lineages with admixture patterns at their boundaries. Approximate Bayesian computation analyses and linear models indicated that the five lineages likely underwent separate evolutionary histories and can be tracked back to distinct glacial refugia. Lineage differentiation started around the Last Glacial Maximum at three focal areas (western, central and eastern Pyrenees) and extended through the end of the Last Glacial Period in the central Pyrenees, where it led to the formation of two more lineages. Our data revealed no evidence of recent dispersal between lineages, whereas borders likely represent zones of secondary contact following expansion from multiple refugia. Finally, we did not find genetic evidence of sex‐biased dispersal. This work highlights the importance of integrating past evolutionary processes and present‐day gene flow and dispersal dynamics, together with multilocus approaches, to gain insights into what shaped the current genetic attributes of amphibians living in montane habitats.     

Rapid Ecological Shift Following Piscivorous Fish Introduction to Increasingly Eutrophic and Warmer Lake Furnas (Azores Archipelago,Portugal): A Paleoecological Approach

Lake ecosystems are nowadays often subjected to multi-stressors, such as eutrophication, climate change, and fish manipulations, the effects of which can be difficult to disentangle, not least from the usual short-term limnological time-series that are available. However, multi-proxy paleoecological approaches may offer such opportunities, especially in the study of remote island lakes characterized by being species poor and buffered somewhat against the recent climate change. We used a paleoecological multi-proxy approach to determine the relative importance of nutrient loading, meteorological forcing, and fish species introduction for recent lake ecosystem development in Lake Furnas on the island of São Miguel, the Azores. The lake was stocked with cyprinids in the late nineteenth century and recently also with piscivorous fish, and has been affected by increasing agricultural activities in its catchment. We analyzed marker pigments, cladoceran remains, and subfossil diatoms in a 46-cm core representing the last 40 years. Remains of large-bodied cladocerans were virtually absent until the introductions of piscivorous pike and pikeperch in 1980 and 1982, respectively, after which the zooplankton community composition changed abruptly. First Ceriodaphnia sp. appeared (ca. 1980), followed by Daphnia a few years later. Carotenoids from cyanobacteria (myxoxanthophyll, aphanizophyll) were regularly present in the lake sediment with a major shift occurring around 1994, from N₂-fixing to non-fixing groups. This shift coincided with the onset of anoxia in the lake water, evidenced by the presence of pigments from Chlorobiaceae (BChl-e homologues and isorenieratene), and with a rapid decrease in benthic-tychoplanktonic diatoms and an increase in cyanobacteria, chlorophytes, dinoflagellates, and cryptophytes. The composition of microbial and algal assemblages changed rapidly after Daphnia appearance, and the covariance between fish stocking, nutrient loading, and enhanced temperatures captured most of the variability in algae accumulation, and thus likely in lake primary production as well. Thus, lake production apparently did not respond strongly to specific changes in temperature, food-web structure, or nutrient input, but rather to the combined effects of all the three forcing mechanisms, emphasizing the role of multi-stressors in lake ecosystem functioning. Our study demonstrates the sensitivity of these remote species-poor lakes to increased nutrient loading, introduction of non-native species, such as fish and climate change.     

Discovering hidden biodiversity: the use of complementary monitoring of fish diet based on DNA barcoding in freshwater ecosystems

Ecological monitoring contributes to the understanding of complex ecosystem functions. The diets of fish reflect the surrounding environment and habitats and may, therefore, act as useful integrating indicators of environmental status. It is, however, often difficult to visually identify items in gut contents to species level due to digestion of soft‐bodied prey beyond visual recognition, but new tools rendering this possible are now becoming available. We used a molecular approach to determine the species identities of consumed diet items of an introduced generalist feeder, brown trout (Salmo trutta), in 10 Tasmanian lakes and compared the results with those obtained from visual quantification of stomach contents. We obtained 44 unique taxa (OTUs) belonging to five phyla, including seven classes, using the barcode of life approach from cytochrome oxidase I (COI). Compared with visual quantification, DNA analysis showed greater accuracy, yielding a 1.4‐fold higher number of OTUs. Rarefaction curve analysis showed saturation of visually inspected taxa, while the curves from the DNA barcode did not saturate. The OTUs with the highest proportions of haplotypes were the families of terrestrial insects Formicidae, Chrysomelidae, and Torbidae and the freshwater Chironomidae. Haplotype occurrence per lake was negatively correlated with lake depth and transparency. Nearly all haplotypes were only found in one fish gut from a single lake. Our results indicate that DNA barcoding of fish diets is a useful and complementary method for discovering hidden biodiversity.     

Using invertebrate remains and pigments in the sediment to infer changes in trophic structure after fish introduction in Lake Fogo: a crater lake in the Azores

Fish introduction may have marked effects on the trophic dynamics and ecological state of former fishless lakes, but due to scarcity of historical data this can seldom be documented. We used remains of cladoceran, chironomid and pigment assemblages in the sediment archive to unravel the effect of introduction of carp (Cyprinus carpio), rainbow trout (Oncorhynchus mykiss) and a cyprinid (Chondrostoma oligolepis) in Lake Fogo, the Azores (Portugal). The stratigraphical record showed two major shifts in community assemblage coinciding with the time of introduction of carp (AD ca. 1890) and trout (AD 1941), respectively. Carp introduction was followed by an abrupt and major decline in the abundance of chironomids, a shift in the cladoceran community from a benthic to a more pelagic dominated community, and Daphnia size was significantly reduced. Pigment assemblages also indicated a shift from a benthic to a pelagic dominated ecosystem, as cryptophytes became markedly more abundant at the expense of benthic diatoms. Trout introduction was followed by a return to a more benthic cladoceran and benthic algae (pigments) dominated state, which we attribute to trout predation on carp leading to improved water clarity. A steady increase in the abundance of pigments and cladoceran remains followed, suggesting enhanced productivity, which may be attributed to enhanced atmospheric nitrogen deposition and introduction of C. oligolepis. We conclude that fish introduction has profoundly altered the trophic dynamics and the relative importance of benthic and pelagic production in this species poor and natural fishless lake in the Azores, and likely in most others lakes at the archipelago islands as fish stocking has been a widespread practice.     

Minnow introductions in mountain lakes result in lower salmonid densities

Biological Invasions - Small fish species such as minnows (Phoxinus sp.) are introduced into mountain lakes by anglers that use them as live bait for fishing salmonids that were previously...