Liliana García

A multimetric diatom index to assess the ecological status of coastal Galician rivers (NW Spain)

There are many rivers in northwest Spain as a consequence of the mountainous landscape and the granitic geology subjected to Atlantic influences. Water and epilithic diatoms samples were collected at 72 sites in Galicia flowing into the Atlantic Ocean and Cantabrian Sea in summer 2002–2003 and spring 2004. These sites included minimally disturbed sites, defined as reference sites, and impacted sites which were influenced by different human pressures. We used the diatom assemblages to calculate diatom indices using the Omnidia software, but we also developed new metrics based on the similarity of species composition in reference sites. The response of the metrics was tested in relation to physicochemical variables. We developed a diatom multimetric index (MDIAT) as a combination of metric values. The sensitivity of the MDIAT to organic and nutrient stressors supports the use of this index to classify the ecological status of Galician rivers. The MDIAT showed higher correlations with some variables and nutrients than the individual metrics. According to the MDIAT, 69% of the sites sampled in Galician coastal rivers achieve good ecological status. The MDIAT has been developed specifically for Galician granitic rivers (NW Spain), and has been intercalibrated at the European level in the Central Baltic Rivers GIG. Our study validates the application of this multimetric index to evaluate the water quality in coastal Galician rivers.

On the way to overcome some ecological riddles of forested headwaters

Life cycles of many detritivores are synchronized to the autumnal input of leaf material in temperate forested headwaters, though some conditions that occur at this season does not seem the most appropriate for an optimal development and growth of individuals. We hypothesized that spring–summer conditions characterized by high temperature and low discharge would support larger numbers of invertebrate individuals inhabiting leaf packs, mostly shredders, and thus larger productivity values. We estimated the production of a dominant detritivore, the chironomid Brillia bifida (Kieffer, 1909), on habitats that represent their specific resource (i.e., leaf packs with different quality) on a seasonal basis that accounted for the high variability of these ecosystems. Our results showed that shredders dominated in numbers (45.8%) during late spring, with B. bifida individuals representing up to 91.7%, mostly on deciduous leaves such as alder, although the individual body size was higher in autumn–winter than in late spring. A laboratory experiment was conducted to complement our field results, and to test only the effects of water temperature and food quality on the development and growth of B. bifida. Our laboratory experiment confirmed the importance of temperature and food quality as main controls on growth–developmental parameters. These controls could strongly affect the ecological strategy of reproduction and colonization of this key detritivore, and ultimately its secondary production.

Negative effects of stagnation and drought on benthic invertebrate communities in lowland streams

Streams are extremely vulnerable to water abstraction across the world because of increasing water demand from humans, as well as because precipitation is decreasing in many areas. To determine how water abstraction affects water chemistry, hydromorphological variables and invertebrate assemblages, we conducted an experiment in which we mimicked two levels of disturbance: stagnation and drought. The experiment was performed at two lowland streams in Galicia (north-west Spain), which were similar in physical conditions but differed in trophic status (high v. low P). Samples were taken both before and after manipulation at the upstream control and downstream-disturbed stretches. There was a significant overall effect of water abstraction on both disturbed stretches, but invertebrate assemblages responded differently between streams and within stretches. In the low-P stream, invertebrate densities remained unchanged in the drought stretch but declined in the stagnation stretch relative to the control. At the same time, the high-P stream exhibited a strong loss of diversity in both the stagnation and drought stretches. These results suggest that short-term flow reductions driven by increasing water scarcity and abstraction put benthic communities in lowland streams at risk, and that risk would be greater (in terms of biodiversity loss) in streams that are initially impaired by high-P loading.

Water abstraction in small lowland streams: Unforeseen hypoxia and anoxia effects.

Flow reduction generated by water abstraction can alter abiotic and biotic properties of stream ecosystems. We hypothesized that reducing stream flow will reduce oxygen levels affecting sensitive invertebrates. We experimentally suppressed flow with longitudinal barriers in two lowland streams of mesotrophic and eutrophic status In each stream we fixed an upstream free flowing control and two downstream disturbed stretches without flow: an initial stagnation stretch and a final drought stretch separated from the stagnation by sand bags to force a greater lowering of the water level. Invertebrates were sampled in control and disturbed stretches before and after the experimental setup for 10weeks, and temperature and oxygen were recorded with data loggers. Flow reduction caused a significant decrease in oxygen, resulting in hypoxia (<4mg O2/L) in the stagnation stretches and anoxia (0mg O2/L) in the drought stretches mainly at night, without influencing water temperature. Invertebrate responded with differential sensitivity to flow and oxygen reduction, some indicator taxa declined at 7.3mg O2/L, others at 6.3mg O2/L, while at 5.3mg O2/L many taxa were severely reduced. Flow reduction generated oxygen depletion, reducing rheophilous and oxygen dependent taxa, while favouring tolerant limnophilous taxa with atmospheric respiration. Passive filterers and scrapers were significantly reduced. Our results indicate that flow reduction can cause hypoxia and anoxia in lowland streams and is an unforeseen effect not addressed in the assessment of flow reduction impacts to streams. Further research is required to evaluate if spatially extensive flow reductions and hypoxia result in long-term impairment of stream biodiversity and function.

Effects of landuse intensification on stream basal resources and invertebrate communities

Human population growth and development have degraded freshwater ecosystems through direct negative effects on basal resources and higher trophic levels. Knowledge of how basal resources are influenced by anthropogenic activities is needed to examine foodweb responses and to evaluate negative effects of human disturbance. We combined quantitative sampling of all putative resources and consumers and naturally occurring stable isotopes of resource C and N to investigate how land use influences characteristics of basal resources (leaf litter, fine particulate organic matter [FPOM] suspended in water, FPOM deposited in pools, and epilithon) and the invertebrate communities in 9 streams of British Columbia (Canada) on a seasonal basis. Study streams were grouped as forest, cropland, and urban, based on percentages of land use in their catchments. We tested for seasonal and spatial differences in biomass, isotopic variability, and overlap of different basal resources and their relationships with invertebrate communities. Our results show that landuse intensification promotes wholesale degradation, which includes deterioration of water quality, changes to the quantity and quality of basal resources, and shifts in community structure of benthic invertebrates. Stream basal resources showed large variations in isotopic signatures caused by landuse changes, with the largest isotopic variability observed in cropland streams and the smallest in urban streams. Invertebrate communities had fewer taxa and reduced functional diversity in impaired streams (i.e., cropland and urban) via simplification of feeding styles of consumers, probably because of bottom-up controls. Our study highlights the importance of quantifying the isotopic variability of basal resources to estimate effects on food webs over time and along landuse intensification gradients.