Jesús Pozo

Continental-scale effects of nutrient pollution on stream ecosystem functioning.

Reading the Leaves Excess inputs of nutrients—a type of pollution known as eutrophication—threatens biodiversity and water quality in rivers and streams. Woodward et al. (p. 1438; see the Perspective by Palmer and Febria) studied how one key ecosystem process—leaf-litter decomposition—responds to eutrophication across a large nutrient pollution gradient in 100 European streams. Leaf breakdown was stimulated by low to moderate nutrient concentrations but was inhibited at high rates of nutrient loading. Leaf-litter breakdown rates across 100 European streams offer insights into ecosystem health during eutrophication. Excessive nutrient loading is a major threat to aquatic ecosystems worldwide that leads to profound changes in aquatic biodiversity and biogeochemical processes. Systematic quantitative assessment of functional ecosystem measures for river networks is, however, lacking, especially at continental scales. Here, we narrow this gap by means of a pan-European field experiment on a fundamental ecosystem process—leaf-litter breakdown—in 100 streams across a greater than 1000-fold nutrient gradient. Dramatically slowed breakdown at both extremes of the gradient indicated strong nutrient limitation in unaffected systems, potential for strong stimulation in moderately altered systems, and inhibition in highly polluted streams. This large-scale response pattern emphasizes the need to complement established structural approaches (such as water chemistry, hydrogeomorphology, and biological diversity metrics) with functional measures (such as litter-breakdown rate, whole-system metabolism, and nutrient spiraling) for assessing ecosystem health.

Inputs of Particulate Organic Matter to Streams with Different Riparian Vegetation

To test whether afforestation with eucalyptus affects the amount, quality, and timing of litter inputs, we evaluated the inputs of coarse particulate organic matter (CPOM) to 2 headwater streams, one flowing through a mixed deciduous forest and one through a plantation of Eucalyptus globulus. Vertical and lateral traps were sampled at least monthly, and the material collected was sorted into 4 main categories: fallen leaves (several species), fruits and flowers, twigs and bark, and debris. The eucalyptus plantation resulted in a reduction of 32% of total litter inputs, and 73% of lateral inputs, compared to the mixed deciduous forest. It also had a less marked seasonal pattern of inputs, and different timing of peak litterfall, the peak in the deciduous forest occurring in autumn, whereas in the eucalyptus plantation the peak occurred in summer. Similarly, leaf diversity was greatly reduced under the eucalyptus, with 98% of the leaf litter mass from 1 species, resulting in broad changes in litter quality. Litter inputs in the eucalyptus plantations contributed 67% less nitrogen and 65% less phosphorus to the stream than in the deciduous forest. Although eucalyptus plantations result in broad changes in organic inputs, no drastic impacts on the structure or function of benthic communities are evident so far in the Agüera stream. Nevertheless, caution suggests maintaining the quantity and timing of allochthonous inputs by keeping buffer strips of native riparian forest.

Effects of Eucalyptus Plantations on Detritus, Decomposers, and Detritivores in Streams

Vast areas of the Iberian Peninsula are covered by monocultures of the exotic tree Eucalyptus globulus. Given that (1) leaf litter produced in the riparian areas is the main energy source for small streams, and (2) trees differ in their nutrient content, chemical defenses, and physical attributes, eucalypt plantations have the potential to affect the biology of streams. Research teams from the University of Coimbra and the University of the Basque Country have been addressing the potential effects of eucalypt plantations at several levels of study. Here we review the main conclusions of these investigations. Eucalypt plantations produced less litter than some deciduous forests. However, there were marked differences in timing of litterfall: litter production peaked during autumn in deciduous forests, whereas in the eucalypt forests it tended to peak in summer and to be more evenly distributed throughout the year. Despite these differences, the average standing stock of organic matter was higher in the eucalypt than in the deciduous forest. This may be attributed to (1) the occurrence of spates or heavy rain in autumn, the period of maximum litter fall in deciduous forests, and (2) bark accumulation in eucalypt forests. Because of differences in leaf composition, the nutrient input in eucalypt forests seems to be lower than in deciduous forests. The rate of decomposition of eucalypt leaves was strongly dependent on nutrients in the water: in nutrient-poor waters it was slower than that of most other leaf species, whereas in nutrient-rich waters it can be as fast as alder – a fast-decaying species. The biomass and cumulative diversity of aquatic hyphomycetes colonizing leaves did not differ between eucalypt and other native leaf species, but fungal sporulation generally peaked 2 weeks later on eucalypt leaves. This lag disappeared when lipids (but not polyphenolics) were chemically removed from eucalypt leaves. Similarly, addition of eucalypt oils to culture media retarded or suppressed fungal growth. Streams bordered by Eucalyptus had lower diversity of fungal spores (but similar spore densities) in Portugal; less consistent patterns were found in similar experiments in Spain. Eucalyptus leaves proved to be poor food for shredders. Under laboratory conditions leaves of Eucalyptus ranked low in food selection experiments using native shredders. The same shredders failed to grow and died when fed exclusively eucalypt leaves. The removal of oils from eucalypt leaves resulted in increased feeding rates, whereas the transfer of oils to alder leaves resulted in decreased feeding rates. The effect of eucalypt plantations on stream invertebrate communities is not very consistent. In nutrient-poor waters, fewer invertebrates colonized eucalypt than alder leaves, but this effect was mitigated after a microbial conditioning period in nutrient-rich waters. Portuguese streams bordered by Eucalyptus had lower numbers of invertebrates than streams surrounded by deciduous forests. In Spanish streams differences were less marked and nonexistent when looking at the composition of the communities, which change more from year to year than from site to site. Most of the eucalypt streams studied in Portugal and Spain dried up in summer, a fact that might reflect an increase in soil hydrophobity produced by Eucalyptus plantations. The very short planting-to-harvest period of eucalypt plantations results in additional impacts, such as soil loss, siltation of streams, or reduced amounts of woody debris in stream channels, which affects their capacity to retain leaf-litter, as well as the availability of habitat for invertebrates and fish. The studies by the Portuguese and Spanish research teams confirm the importance of maintaining riparian buffer strips to reduce human impact on streams and rivers.

Environmental controls of whole-stream metabolism identified from continuous monitoring of Basque streams

Abstract Most methods for assessing the ecological status of streams focus on structural characteristics (water quality, community composition, riparian vegetation) but neglect functional properties of the ecosystem because routine methods to assess stream function are scarce. Metabolism, one of the most integrative ecosystem functions, can be a good indicator of stream function because it is relevant across all sizes and types of streams, is sensitive to stressors, such as eutrophication or changes in riparian cover, and can be measured continuously. Environmental controls on whole-ecosystem metabolism were measured at 19 contrasting stream reaches in the Basque Country (northern Spain). Discharge, temperature, and O2 were monitored continuously for 15 mo, reaeration rate was calculated with the nighttime regression method, and whole-stream metabolism was calculated by the single-station open-channel method. The effect of discharge on reaeration coefficients was highly site-specific. Average gross primary production (GPP) ranged from 2.7 to 11.0 g O2 m−2 d−1, was highest at eutrophic sites, and showed no relationship with periphyton biomass. Ecosystem respiration (ER) ranged from 6.3 to 42.6 g O2 m−2 d−1 and was highest at polluted sites. Differences among sites increased in summer. All sites were heterotrophic on an annual basis, but 3 were autotrophic during summer. Turbidity was the main controller of primary production during summer and explained 20% and 39% of the spatial variation in GPP and net ecosystem production, respectively. Biological O2 demand of water explained 40% of ER variance. Catchment activities also controlled GPP, which decreased as population density increased. To our knowledge, our study is the first report of continuous monitoring of whole-stream metabolism at many reaches simultaneously, and it shows the potential of this technique for routine monitoring of stream function.

Afforestation with Eucalyptus globulus and leaf litter decomposition in streams of northern Spain

To test the hypothesis that decomposition of leaf species in streams is influenced by afforestation with Eucalyptus globulus, we compared decay rates, nutrient levels, fungal biomass and macroinvertebrate assemblages on alder and eucalyptus leaf litter in three streams (two headwaters under different forests, and a mid reach) of the Aguera catchment (northern Spain). Whatever the reach, alder always decomposed significantly faster than eucalyptus. Litter contents in nitrogen and phosphorus rose during breakdown at the mid reach, but not at the headwaters. No differences in fungal biomass were found between alder and eucalyptus leaves at the headwater reaches; however, at the mid reach, eucalyptus showed the highest values. Alder litter, a high quality substratum, was readily colonized by shredders, and decayed rapidly at all sites. Eucalyptus, a low quality species, had lower nutrient contents and was less favoured by shredders. Under high nutrient levels (particularly phosphorus), however, it was readily colonized by fungi, thus shifting from medium to high breakdown rates. The potentially negative impact of afforestation with eucalyptus on streams can thus be reduced in situations of high concentrations of dissolved nutrients.

Effect of removal of wood on streambed stability and retention of organic matter

We tested the hypothesis that wood influences stream channel morphology, sediment composition, retention, and storage of organic matter by experimentally removing all wood from 2 first-order reaches (ca 90 m length) of 2 neighboring tributaries (Salderrey and Cuchillo streams) in the Agüera catchment (Basque Country, Spain). We established 2 control reaches upstream from these treatment reaches. We completed maps of substrate, fill/scour transects, and wood surveys in 1997 (prior) and 1998 (after) wood removal. We measured monthly inputs of fine wood to the treatment reaches. In addition, we measured seston every 2 wk, benthic coarse organic particulate matter (CPOM) every 2 mo, and the retention capacity of reaches every 3 mo. All reaches were scoured during the study period, but the volume of sediment lost was higher in the treatment reaches (53 m3) than in the controls (14.2 m3 in Salderrey, 2.7 m3 in Cuchillo). As a result, the area of coarse substrate increased in the treatments, but remained unaltered in the controls. The capacity of the reaches to retain CPOM decreased after the treatment, but affected neither seston concentration nor the benthic storage of CPOM. Wood is an important constituent of Basque streams, and removal of wood significantly impacts channel structure and organic matter storage.

Eucalyptus plantations affect fungal communities associated with leaf-litter decomposition in iberian streams

The replacement of diverse deciduous forests by eucalyptus plantations changes the timing, quality and quantity of litter inputs to streams, which has the potential to affect the activity of decomposers and thus ecosystem functioning. Here, we compared (a) the decomposition rate of alder and oak leaves incubated in deciduous and eucalyptus streams in Spain and Portugal, (b) the activity (fungal biomass and sporulation) and diversity (species richness and Pielous evenness index) of the associated fungal communities and (c) changes in N and P content of leaves. Alder and oak leaves decomposed at similar rates in both stream types and countries, with the exception of oak leaves in the Spanish eucalyptus stream, which decomposed faster than in the corresponding deciduous stream or in the Portuguese eucalyptus stream. This difference was attributed to physical fragmentation due to flooding and not to forest cover. Higher nitrogen and phosphorus content and higher fungal biomass and sporulation were generally found on leaves from eucalyptus rather than from deciduous streams. The higher fungal activity in eucalyptus streams was attributed to higher water temperature and benthic organic matter storage. The Spanish eucalyptus stream had higher species richness of aquatic hyphomycetes than the deciduous one (27 vs. 20) while in Portugal the opposite was true (16 vs. 20). Fungal community evenness was significantly higher on alder leaves in eucalyptus than in deciduous streams. The community structure (MDS analysis) discriminated both stream types in Portugal much better than it did in Spain. At least for Portugal, differences between stream types can be explained by higher litter diversity in deciduous than in eucalyptus streams. In conclusion, stream fungal communities in Portugal were more affected by eucalyptus plantations than in Spain. In both countries, fungal diversity and activity were more affected by eucalyptus plantations than decomposition rates of submerged litter. We suggest therefore that, to mitigate the effect of eucalyptus plantations, deciduous trees could be planted on the river banks or, preferably, riparian strips of native vegetation should be left unmodified.

Impact of a eucalyptus (Eucalyptus globulus Labill.) plantation on the nutrient content and dynamics of coarse particulate organic matter (CPOM) in a small stream

Litterfall inputs, benthic storage and the transport of coarse particulate organic matter (CPOM) were studied in two headwater streams, one flowing through a mixed deciduous forest and one through a plantation of Eucalyptus globulus. Vertical and lateral traps, transported CPOM and benthic CPOM were sampled monthly to biweekly and sorted into four categories: leaves, twigs and bark, fruits and flowers and debris. The litterfall inputs were about 20% lower at the eucalyptus site but this reduction was unevenly distributed among the litter categories. The reduction of the nitrogen and phosphorus inputs was larger (50%) than that of CPOM because of the low nutrient concentration of the CPOM at the eucalyptus site. Transported CPOM was also lower at the eucalyptus site. Although total CPOM inputs to the stream were reduced in the eucalyptus plantation, benthic storage of CPOM was 50% higher due to (1) high inputs of CPOM and low discharge during summer, (2) more twig and bark inputs, (3) eucalyptus leaves being retained more efficiently in the stream than deciduous leaves (4) a lower discharge, which may in part be attributable to eucalyptus-induced changes in the hydrological cycle. Increased retention balanced lower nitrogen and phosphorus content of CPOM, so benthic storage of nitrogen and phosphorus was similar at both sites. This work demonstrates that the timing, quality and quantity of inputs and benthic storage of CPOM in streams changes substantially because of the substitution of natural deciduous forest with eucalyptus plantation. Maintenance of buffer strips of natural vegetation may be the best way to protect ecological functioning of small, forested streams.

Leaf retention in streams of the Agüera basin (northern Spain)

Abstract. The capacity of stream channels to retain leaf litter (retentiveness) was measured in 21 reaches of the Agüera basin (northern Spain) at different discharges, using plastic strips as leaf analogs. Strips were calibrated against seven local leaf species occurring in the area. Retention was highest for alder, followed by plastic strips, oak, beech, chestnut, eucalyptus, hazel, and sycamore. Inter-specific differences in retention were great, and not clearly related to leaf form or size. This result shows that a great deal of caution is necessary to compare results obtained by authors using different leaf species. The Agüera stream channels were highly retentive, especially in the headwaters. At baseflows, the average travel distance of strips was 3.6 m in 1st-order reaches, increasing to 16.6 m in 3rd-order streams. Travel distances of strips increased twofold in 3rd- and 2nd-order reaches and 5-fold in 1st-order streams during periods of high discharge. Leaf litter retentiveness was related to channel gradient, width, and substrate. Cobbles and wood showed high retention efficiencies, and the role of wood as a retention factor increased at high discharges. Retentiveness enhances storage and subsequent utilization of organic materials in forested streams, and thus should be taken into account when managing streams.

Stream regulation by small dams affects benthic macroinvertebrate communities: from structural changes to functional implications

We studied benthic macroinvertebrate communities upstream and downstream of five small reservoirs (surface release in autumn–winters) (north Spain) to assess the effect of flow regulation on structural and functional characteristics of stream ecosystems. We based our approach on the use of structural metrics (density, biomass, richness and diversity) in combination with two functional diversity indices based on biological and ecological traits: FDPG index, related to species richness, and FDQ, which incorporates evenness across taxa. Although water physicochemical parameters were unaffected by the reservoirs during the study period (autumn–winter), macroinvertebrate metrics were lower below the dams, with detritivores (shredders and collector-gatherers) being the most affected. The alder leaf breakdown rate estimated by the litter-bag technique was related to the density, biomass, richness, diversity and FDPG index of shredders, compromising the ecosystem functioning. The most plausible origin for the observed differences in macroinvertebrate metrics between upstream and downstream reaches was the change of the flow regime caused by the impoundments at downstream sites, leading to droughts in summer in those naturally permanently flowing streams. The observed functional diversity loss might reduce the chances of the community to override natural or man-induced fluctuations in their environment with possible repercussions on important ecosystem functions and services.