Adonis Giorgi

Drought and postdrought recovery cycles in an intermittent Mediterranean stream: structural and functional aspects

Abstract The effects of the intensity of seasonal droughts on stream ecosystems were studied in the Fuirosos, an intermittent forested Mediterranean stream. Macroinvertebrate community structure and stream ecosystem metabolism were measured during seasonal summer droughts in 2001, 2002, and 2003. Ecosystem metabolism was profoundly affected by stream intermittency. Organic matter that accumulated during the dry period enhanced ecosystem respiration during the postdrought recovery. Highest biotic diversity was found at low water levels as the stream dried and contracted. Macroinvertebrate community response to drying was stepped and apparently defined by thresholds of transition from drying to cessation of flow and from the dry phase to restoration of flow. Environmental conditions changed markedly with cessation of flow, causing large changes in community structure during 2001 and 2003 (dry years). Drying caused an increase in macroinvertebrate density that peaked in isolated pools soon after flow ceased, but then decreased rapidly because of the physicochemical changes associated with fragmentation of the watercourse. The macroinvertebrate community at the end of the summer dry phase (when flow resumed) differed from the community that had been present before drying began. Differences in community structure during the summer dry period were not as marked in 2002 (a wet year) as in 2001/2003. The influence of drought on the macroinvertebrate community differed across substrata. Drying led to significant changes in density on cobbles and leaves, but not on sand. Few taxa resisted drying, and resilience to drying was the dominant response to disturbance in the Fuirosos.

The influence of substratum type and nutrient supply on biofilm organic matter utilization in streams

We investigated the effect of benthic substratum type (sand and rocks) and nutrient supply (N and P) on biofilm structure and heterotrophic metabolism in a field experiment in a forested Mediterranean stream (Fuirosos). Rock and sand colonization and biofilm formation was intensively studied for 44 d at two stream reaches: control and experimental (continuous addition of phosphate, ammonia, and nitrate). Structural (C, N, and polysaccharide content and bacterial and chlorophyll density) and metabolic biofilm parameters (b-glucosidase, peptidase, and phosphatase enzyme activities) were analyzed throughout the colonization process. The epilithic biofilm (grown on rocks) had a higher peptidase activity at the impacted reach, together with a higher algal and bacterial biomass. The positive relationship between the peptidase activity per cell and the N content of the epilithic biofilm suggested that heterotrophic utilization of proteinaceous compounds from within the biofilm was occurring. In contrast, nutrient addition caused the epipsammic biofilm (grown on sand) to exhibit lowerb-glucosidase and phosphatase activities, without a significant increase in bacterial and algal biomass. The differential response to nutrient addition was related to different structural characteristics within each biofilm. The epipsammic biofilm had a constant and high C : N ratio (22.7) throughout the colonization. The epilithic biofilm had a higher C : N ratio at the beginning of the colonization (43.2) and evolved toward a more complex structure (high polysaccharide content and low C : N ratio) during later stages. The epipsammic biofilm was a site for the accumulation and degradation of organic matter: polysaccharides and organic phosphorus compounds had higher degradation activities.

Meteorological and riparian influences on organic matter dynamics in a forested Mediterranean stream

Abstract Organic matter inputs, transport, and storage, ecosystem metabolism, and organic C turnover length were measured in a forested Mediterranean 3rd-order stream (Fuirosos) over a period of 3 y. Meteorological patterns influenced the organic matter dynamics of Fuirosos through 2 pathways: summer weather and flood pulses. Summer rains affected the timing of litter fall and the extent of flow intermittency, which gave rise to accumulations of organic matter in the streambed in dry years. With the onset of flow, these organic matter accumulations were the energy sources for considerable ecosystem respiration (30 g O2 m−2 d−1). Interflood periods punctuated by flood pulses determined cycles of steady accumulation and abrupt removal of stored organic matter. During the interflood periods, the efficiency of organic matter processing increased continuously. The seasonal changes in the riparian forest influenced the ecological consequences of the flood pulses. Resilience of the ecosystem, measured in terms of gross primary production and ecosystem respiration, strongly depended on the supply of benthic organic matter and light availability, both of which were seasonally variable in the deciduous riparian forest.

Effects of nutrient inputs in a forested mediterranean stream under moderate light availability

Nutrients were artificially increased in an oligotrophic, forested stream by adding nitrogen (as nitrate and ammonia) and phosphorus (as reactive phosphate) continuously for 44 days. This addition increased the nutrient concentrations by 10 (phosphorus), 6 (ammonia) and 4 times (nitrates) with respect to basal concentrations. The effect of the enrichment on the structure and metabolism in benthic habitats (rocks, sand and leaf litter) was studied during a period of moderate light availability (maximum light at the streambed ranged between 20-120 μE m -2 s -1 ), prior to leaf occurrence in the riparian vegetation. A BACI design was used to determine the significance of the addition by comparing an upstream non-enriched (control) reach with a downstream enriched one (impacted). Nutrient addition caused a 3-fold increase in chlorophyll density on rocks and sand, and a remarkable increase in algal cell density, but did not cause changes in the community composition of the algal community. The increase in algal biomass was not reflected in higher net community metabolism (NCM) when this was measured as oxygen production, but higher carbon incorporation was detected at the end of the experiment. Nutrient addition did not significantly alter bacterial cell density on rocks, sand and leaf litter substrata. Accordingly, no substantial increase in community respiration (CR) was detected. Neither did heterotrophic activity (extracellular enzymes) change significantly because of addition, except for a decrease (although weak) in phosphatase in sand. Enrichment did not cause a general increase in macroinvertebrate density or biomass, but some taxa increased in the enriched reach. Scrapers (Ancylus fluviatilis) and detritivores (Oligochaeta, Lumbriculidae significantly augmented their density or biomass on certain substrata. Overall, enhanced nutrient availability was able to cause a remarkable biomass increase in the autotrophic component of the stream, in spite of the moderate light availability.

As productive and slow as a stream can be—the metabolism of a Pampean stream

Abstract Stream metabolism at both ecosystem and functional-compartment scales was measured in a low-order Pampean stream (La Choza) over a 3-wk period to characterize metabolic rates and discern the contribution of each functional compartment (submerged macrophytes, benthos, floating macroalgae, water column, and hyporheic zone) to ecosystem metabolism. La Choza stream is an autotrophic ecosystem during low flows and has gross primary production rates of up to 22 g O2 m−2d−1, which are among the highest reported in the literature and set an upper bound on how productive streams can be in the absence of light and nutrient limitations. Floating macroalgae provided most of the primary production (30–90%), whereas the hyporheic zone provided most of the ecosystem respiration (40–80%). The differential effects of high flows on the different functional compartments depressed the production∶respiration ratio, suggesting a strong relationship between flow and metabolism. Thus, low flows enhanced primary production and led to diel dissolved O2 concentration oscillations between 0 and 25 g O2/m3. In contrast, high flow depressed primary production by an order of magnitude and increased ecosystem respiration. High production rates during the low-flow period and extreme physicochemical conditions (anoxia for 7–8 h on a daily basis) may be typical in this type of ecosystem during extended low-flow periods.

Temporal and spatial variability in streams of a pampean basin

The Argentine pampean streams exist in a traditionally agricultural region, but they also receive a supply of contaminants in the more urban zones. The objectives of this study are:1. To analyze the physical, chemical and microbiological characteristics of the streams in the Luján river basin; and2. To describe the annual variation of some chemical characteristics (primarily nutrients) in one of the streams.Two samplings (winter and summer) were made in 20 streams to determine some physical, chemical and microbiological variables. Topographical variables were also estimated in each stream. Additionally, in one of the streams (Las Flores), monthly measurements of current velocity, flow, and dissolved nitrogen, phosphorus and oxygen were taken during 4 years.The streams are characterized by high nutrient contents. There is no strong heterogeneity in stream chemical characteristics within the same period, however seasonal variations are important . The principal components analysis suggests the existence of two axes of variation within the basin. One reflects a pollution gradient that increases downstream; the other axis shows that waters change from high to low alkalinity and from less to more eutrophic.In Las Flores stream, nutrient concentration varied greatly between years. The concentration of soluble reactive phosphorus was significantly correlated with stream flow (P < 0.05). Though there was no correlation between nitrate levels and flow, some estimates indicated that these ions are in high concentrations in the groundwater.The physiognomical characteristics of pampean streams differ greatly from those described in temperate streams of the northern hemisphere, and it may be assumed that they also exhibit distinct functional characteristics.

Global changes in pampean lowland streams (Argentina): implications for biodiversity and functioning

The rivers and streams in the pampean plains are characterized by a low flow rate due to the low slope of the surrounding terrain, high levels of suspended solids, silty sediment in the benthos, and reduced rithron; the riparian forest of this region has been replaced by low-altitude grasslands. Many of these environments contain a wide coverage of aquatic reeds, both submerged and floating, making the pampas limologically extraordinary. These terrains have undergone a gradual transformation in response to the progress of urbanization and agricultural activity in recent years with a resulting loss of biodiversity, leaving only few sites that continue to reflect the original characteristics of the region. Because of human activities in combination with the global climate change, variations have occurred in biological communities that are reflected in the structure and function of populations and assemblages of algae, macrophytes, and invertebrate fauna or in the eutrophication of affected ecosystems. The objective of this article is to describe the principal limnologic characteristics of the streams that traverse the Buenos Aires Province and relate these features with the predicted future global changes for the area under study. Considering the future climate-change scenarios proposed for the pampean region, the projected increment in rainfall will affect the biological communities. Higher rainfall may enhance the erosion and generate floodings; increasing the transport of sediments, nutrients, and contaminants to the ocean and affecting the degree of water mineralization. Changes in discharge and turbidity may affect light penetration in the water column as well as its residence time. The modifications in the use of the soil will probably favor the input of nutrients. This latter effect will favor autotrophy, particularly by those species capable of generating strategies for surviving in more turbid and enriched environments. An accelerated eutrophication will change the composition of the consumers in preference to herbivores and detritivores. The increase in global population projected for the next years will demand more food, and this situation coupled with the new scenarios of climate change will lead to profound socioeconomic changes in the pampean area, implying an increase in demand for water resources and land uses.

EFFECT OF AN INDUSTRIAL DISCHARGE ON WATER QUALITY AND PERIPHYTON STRUCTURE IN A PAMPEAM STREAM

Seasonal sampling was carried out at four sites on a pampeanstream that receives industrial effluent from two textile factories. To evaluate water quality, several physical and chemical parameters were examined and the periphyton growing oncattail (Typha latifolia L.) were analyzed.Water quality and periphyton structure differed significantlybetween sites upstream and downstream of the discharge. Differences in temperature and also in concentrations of phosphate, dissolved oxygen, and phaeopigment were detected. At the same time, changes in the dominant algae groups wereobserved. Downstream of the industrial discharge, the numberof Bacillariophyta decreased, while species of Cianophyta andEuglenophyta were more abundant. This abundance correlated withincreased phosphate and organic matter content and decreased oxygen concentration. Although this study did not detect a reduction in the number of species, similarity between stands decreased downstream of the industrial discharge. Changes incommunity structure were readily detected in this situation because the communities of the polluted and unpolluted zones were qualitatively different. Periphyton growing naturally on Typha latifolia is a useful indicator of the impact of waste waters on the biota and can also be used to evaluate water body recovery.

Macroinvertebrates select complex macrophytes independently of their body size and fish predation risk in a Pampean stream

Abstract Macrophyte complexity has been associated with high abundance and richness of macroinvertebrates. While the effect on richness has been attributed to an increase in the number of niches, the effect on abundance has been explained by a higher availability of space for small individuals, refuge, and/or food. For studying effects of complexity on macroinvertebrates, we used complementary approaches of laboratory choice and field colonization experiments, with macrophytes (Egeria densa and Elodea ernstae) and plastic imitations of contrasting fractal dimension. We investigated whether macroinvertebrates may actively select complex habitats by Hyalella sp. choice experiments. Then, we tested effects of complexity on macroinvertebrate density, biomass, richness, diversity, and body size using colonization experiments. Finally, a caging experiment was performed to study interacting effects of complexity and predation. The active choice of complex substrates by Hyalella sp., and the significant positive relationship between macrophyte fractal dimension and macroinvertebrate density support the existence of a positive effect of complexity on abundance. As macroinvertebrate length was not associated with fractal dimension, such differences could not be attributed to a higher space available for smaller invertebrates in complex plants. Finally, neither macroinvertebrate density nor size was reduced by fish predation in the Las Flores stream.

Metabolism in a macrophyte-rich stream exposed to flooding

The aim of this study was to investigate the effect of floods on the metabolic autotrophic rates of a Pampean stream. We hypothesized that there would be high productivity because of the macrophyte-rich community and the high nutrient levels but that this productivity would be reduced by flooding. Net community production (NCP) and community respiration (CR) were measured using clear and opaque acrylic chambers in the same reach of the stream. Community metabolism was analyzed in relation to biomass and the colonized streambed surface. Prior to the flood, epiphyton was the most productive compartment of the stream, whereas after the flood, the bottom algae compartment was the most productive one. Therefore, the relative contribution of each compartment to the entire ecosystem was influenced by the varying flow conditions. The primary gross production values of the Las Flores stream communities before the flood were higher than most of those reported in other streams worldwide and sustain the complex trophic web associated to the stream. Consequently, production decrease due to the lower relative contribution of macrophytes and epiphyton would lead to a more simplified trophic network.