Breakdown and detritivore colonisation of leaves in three New Zealand streams

Breakdown of leaves from three native riparian tree species, and their colonisation by shredding and collecting insect larvae, were investigated in three streams on Banks Peninsula, New Zealand. Leaves were introduced in baskets at the time of leaf fall. Breakdown rates of leaves were faster than previously recorded in New Zealand streams and were comparable to those of many northern hemisphere deciduous species. Shredder and total detritivore densities and biomass in leaf baskets were also greater than previously found in New Zealand streams. Peaks of shredder biomass on red beech and mahoe leaves were found when only about 20% of leaf biomass remained. No shredder peak was recorded on fuchsia leaves, and no collector peaks occurred in any of the streams. Relative shredder and collector biomass (per g DW leaf) in leaf baskets did not exceed or was smaller than in leaf litter accumulations of mixed origin and conditioning throughout the streams during leaf breakdown although absolute shredder and collector biomass (per m² stream bottom) was occasionally larger in baskets than in the rest of the stream. These findings support contentions that spatial and temporal relationships between detrital inputs and detritivore biomass and life histories are weak in New Zealand streams.     

Urban infrastructure influences dissolved organic matter quality and bacterial metabolism in an urban stream network

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Top-down and bottom-up influences on populations of a stream detritivore

1. Knowledge of the influence of predatory fish in detritus‐based stream food webs is poor. We tested whether larval abundance of the New Zealand leaf‐shredding caddisfly, Zelandopsyche ingens (family Oeconesidae), was affected by the presence of predatory brown trout, Salmo trutta and the abundance of their primary detrital resource (Nothofagus leaves). 2. The density of Z. ingens and the biomass of leaves were determined in seven fishless streams and four trout streams in the Cass region, central South Island, on four occasions spanning 5 years. 3. Physicochemical conditions were similar in trout and fishless streams, but ancova indicated that Z. ingens numbers were positively related to leaf biomass and that caddisfly numbers were significantly greater in fishless streams than trout streams for any given biomass of leaf. The cases of trout stream larvae were also heavier per unit length than those in fishless streams. 4. Our results provide evidence for both top‐down and bottom‐up influences on a detritus‐based stream food web. Although stream detritivores may benefit from a habitat that provides both food and a degree of protection from predators, top‐down effects of predators on detritivore population abundance were still important. Thus, detrital resource availability may determine maximum attainable population size, whereas predation is likely to reduce the population to a level below that.     

Nutrients stimulate leaf breakdown rates and detritivore biomass: bottom-up effects via heterotrophic pathways

Most nutrient enrichment studies in aquatic systems have focused on autotrophic food webs in systems where primary producers dominate the resource base. We tested the heterotrophic response to long-term nutrient enrichment in a forested, headwater stream. Our study design consisted of 2 years of pretreatment data in a reference and treatment stream and 2 years of continuous nitrogen (N) + phosphorus addition to the treatment stream. Studies were conducted with two leaf species that differed in initial C:N, Rhododendron maximum (rhododendron) and Acer rubrum (red maple). We determined the effects of nutrient addition on detrital resources (leaf breakdown rates, litter C:N and microbial activity) and tested whether nutrient enrichment affected macroinvertebrate consumers via increased biomass. Leaf breakdown rates were ca. 1.5 and 3× faster during the first and second years of enrichment, respectively, in the treatment stream for both leaf types. Microbial respiration rates of both leaf types were 3× higher with enrichment, and macroinvertebrate biomass associated with leaves increased ca. 2–3× with enrichment. The mass of N in macroinvertebrate biomass relative to leaves tended to increase with enrichment up to 6× for red maple and up to 44× for rhododendron leaves. Lower quality (higher C:N) rhododendron leaves exhibited greater changes in leaf nutrient content and macroinvertebrate response to nutrient enrichment than red maple leaves, suggesting a unique response by different leaf species to nutrient enrichment. Nutrient concentrations used in this study were moderate and equivalent to those in streams draining watersheds with altered land use. Thus, our results suggest that similarly moderate levels of enrichment may affect detrital resource quality and subsequently lead to altered energy and nutrient flow in detrital food webs. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

Towards a budget of leaf litter decomposition in a first-order woodland stream

To construct a budget of carbon transformations occurring during leaf decomposition, alder leaves were placed in a woodland stream, later retrieved at weekly intervals, and rates of fungal and bacterial production, microbial respiration, and release of dissolved organic matter (DOM) and fine particulate organic matter (FPOM) were determined during short laboratory incubations. Carbon dioxide was the major decomposition product, explaining 17% of the microbially mediated leaf mass loss. DOM and FPOM were also important products (5 and 3% of total mass loss, respectively), whereas carbon flow to microbial biomass was low (2%). Fungal biomass in leaves always exceeded bacterial biomass (95–99% of total microbial biomass), but production of bacteria and fungi was similar, indicating that both types of microorganisms need to be considered when examining leaf decomposition in streams. Comparison of leaf mass loss in coarse and fine mesh bags revealed, in addition, that the shredder, Gammarus pulex, had a major impact on leaf decomposition in this study.     

APPLIED ISSUES Increasing litter retention in moorland streams: ecological and management aspects of a field experiment

1. The retention characteristics of two moorland streams in mid-Wales were manipulated for 2 years by the addition of small traps which accumulated detritus. Leaf litter was also added to these essentially treeless streams at regular intervals to simulate natural inputs to a woodland stream. 2. Leaf traps retained a significantly higher biomass of detritus - both naturally occurring and added - than either the surrounding stream bed or unmanipulated reference sections. They also supported significantly higher numbers of nemourid, leuctrid, tipulid and elminthid insect larvae. 3. Among functional feeding groups, detritivores and, to a lesser extent, predators responded to increased detritus availability. Numbers of mayfly larvae were low in leaf packs, suggesting a negative effect of detritus aggregations on their numbers. 4. The taxa which responded positively to increased detrital biomass, particularly stonefly larvae, are known to be tolerant of low pH conditions, whereas those affected detrimentally are generally absent from acid waters. It is proposed therefore that increasing detrital inputs and litter retention in culturally acidified upland streams may serve to increase their invertebrate productivity.     

Dissolved organic matter and phosphorus leached from fresh and 'terrestrially' aged river red gum leaves: implications for assessing river–floodplain interactions

1. The rates of leaching, speciation and bioavailability of dissolved organic matter (DOM) and dissolved phosphorus (P) leached from fresh leaves of the river redgum, Eucalyptus camaldulensis Dehnh, were compared to those from leaves which had been aged on the flood plain for 5_months. The DOM and P leaching rates from microbially inhibited aged leaves were first-order with respect to leaf quantity. The kinetics of DOM and P leaching from fresh leaves were more complex; DOM leaching from fresh leaves appeared to be self-inhibitory, while P leaching from leaves was potentially enzymatically mediated.
2. The speciation and microbial bioavailability of DOM from fresh and aged leaves were completely different. At lower leaf biomass, almost all the DOM from fresh leaves was utilized by an introduced microbial consortium; at higher biomass, microbial utilization appeared to be nutrient limited. Conversely, only about 30% of the DOM leached from aged leaves was utilized by the introduced microbial consortium during the course of the experiment. The difference in microbial utilization could be a result of changes in DOM speciation as a consequence of terrestrial ageing.
3. Weak-anion exchange chromatograms of microbially inhibited fresh leaf extracts showed numerous (unassigned) DOM peaks, most of which could be used by the microbial consortium present. The weak-anion exchange chromatograms of sterile aged leaves showed only three broad peaks and a number of smaller spikes. Only one of the broad peaks could be utilized by the microbial consortium.
4. Phosphorus speciation was also determined by weak-anion exchange chromatography. Most of the P leached from both fresh and aged leaves was free orthophosphate, and therefore, readily available. Two organic-P species leached from microbially inhibited fresh leaves were also found to be readily available to the microbiota.     

Comparison of Fungal Activities on Wood and Leaf Litter in Unaltered and Nutrient-Enriched Headwater Streams

Fungi are the dominant organisms decomposing leaf litter in streams and mediating energy transfer to other trophic levels. However, less is known about their role in decomposing submerged wood. This study provides the first estimates of fungal production on wood and compares the importance of fungi in the decomposition of submerged wood versus that of leaves at the ecosystem scale. We determined fungal biomass (ergosterol) and activity associated with randomly collected small wood (<40 mm diameter) and leaves in two southern Appalachian streams (reference and nutrient enriched) over an annual cycle. Fungal production (from rates of radiolabeled acetate incorporation into ergosterol) and microbial respiration on wood (per gram of detrital C) were about an order of magnitude lower than those on leaves. Microbial activity (per gram of C) was significantly higher in the nutrient-enriched stream. Despite a standing crop of wood two to three times higher than that of leaves in both streams, fungal production on an areal basis was lower on wood than on leaves (4.3 and 15.8 g C m⁻² year⁻¹ in the reference stream; 5.5 and 33.1 g C m⁻² year⁻¹ in the enriched stream). However, since the annual input of wood was five times lower than that of leaves, the proportion of organic matter input directly assimilated by fungi was comparable for these substrates (15.4 [wood] and 11.3% [leaves] in the reference stream; 20.0 [wood] and 20.2% [leaves] in the enriched stream). Despite a significantly lower fungal activity on wood than on leaves (per gram of detrital C), fungi can be equally important in processing both leaves and wood in streams.     

Leaf breakdown in streams differing in catchment land use

1. The impact of changes in land use on stream ecosystem function is poorly understood. We studied leaf breakdown, a fundamental process of stream ecosystems, in streams that represent a range of catchment land use in the Piedmont physiographic province of the south‐eastern United States. 2. We placed bags of chalk maple (Acer barbatum) leaves in similar‐sized streams in 12 catchments of differing dominant land use: four forested, three agricultural, two suburban and three urban catchments. We measured leaf mass, invertebrate abundance and fungal biomass in leaf bags over time. 3. Leaves decayed significantly faster in agricultural (0.0465 day^?1) and urban (0.0474 day^?1) streams than in suburban (0.0173 day^?1) and forested (0.0100 day^?1) streams. Additionally, breakdown rates in the agricultural and urban streams were among the fastest reported for deciduous leaves in any stream. Nutrient concentrations in agricultural streams were significantly higher than in any other land‐use type. Fungal biomass associated with leaves was significantly lower in urban streams; while shredder abundance in leaf bags was significantly higher in forested and agricultural streams than in suburban and urban streams. Storm runoff was significantly higher in urban and suburban catchments that had higher impervious surface cover than forested or agricultural catchments. 4. We propose that processes accelerating leaf breakdown in agricultural and urban streams were not the same: faster breakdown in agricultural streams was due to increased biological activity as a result of nutrient enrichment, whereas faster breakdown in urban streams was a result of physical fragmentation resulting from higher storm runoff.     

Fungal growth, production, and sporulation during leaf decomposition in two streams.

I examined the activity of fungi associated with yellow poplar (Liriodendron tulipifera) and white oak (Quercus alba) leaves in two streams that differed in pH and alkalinity (a hard water stream [pH 8.0] and a soft water stream [pH 6.7]) and contained low concentrations of dissolved nitrogen (<35 microg liter(-1)) and phosphorus (<3 microg liter(-1)). The leaves of each species decomposed faster in the hard water stream (decomposition rates, 0.010 and 0.007 day(-1) for yellow poplar and oak, respectively) than in the soft water stream (decomposition rates, 0.005 and 0.004 day(-1) for yellow poplar and oak, respectively). However, within each stream, the rates of decomposition of the leaves of the two species were not significantly different. During the decomposition of leaves, the fungal biomasses determined from ergosterol concentrations, the production rates determined from rates of incorporation of [(14)C]acetate into ergosterol, and the sporulation rates associated with leaves were dynamic, typically increasing to maxima and then declining. The maximum rates of fungal production and sporulation associated with yellow poplar leaves were greater than the corresponding rates associated with white oak leaves in the hard water stream but not in the soft water stream. The maximum rates of fungal production associated with the leaves of the two species were higher in the hard water stream (5.8 mg g(-1) day(-1) on yellow poplar leaves and 3.1 mg g(-1) day(-1) on oak leaves) than in the soft water stream (1.6 mg g(-1) day(-1) on yellow poplar leaves and 0.9 mg g(-1) day(-1) on oak leaves), suggesting that effects of water chemistry other than the N and P concentrations, such as pH or alkalinity, may be important in regulating fungal activity in streams. In contrast, the amount of fungal biomass (as determined from ergosterol concentrations) on yellow poplar leaves was greater in the soft water stream (12.8% of detrital mass) than in the hard water stream (9.6% of detrital mass). This appeared to be due to the decreased amount of fungal biomass that was converted to conidia and released from the leaf detritus in the soft water stream.     

Detrital processing in streams exposed to acidic precipitation in the Central Appalachian Mountains

Continuing high rates of acidic deposition in the eastern United States may lead to long-term effects on stream communities, because sensitive catchments are continuing to lose anions and cations. We conducted a two-year study of the effects of pH and associated water chemistry variables on detrital processing in three streams with different bedrock geology in the Monongahela National Forest, West Virginia. We compared leaf pack processing rates and macroinvertebrate colonization and microbial biomass (ATP concentration) on the packs in the three streams. Breakdown rates of red maple and white oak leaf packs were significantly lower in the most acidic stream. The acidic stream also had significantly lower microbial and shredder biomass than two more circumneutral streams. Shredder composition differed among streams; large-particle detritivores dominated the shredder assemblages of the two circumneutral streams, and smaller shredders dominated in the acidic stream. Within streams, processing rates for three leaf species were not significantly different between the two years of the study even though invertebrate and microbial communities were different in the two years. Thus, macroinvertebrate and microbial communities differed both among streams that differed in their capacity to buffer the effects of acidic precipitation and among years in the same stream; these differences in biotic communities were not large enough to affect rates of leaf processing between the two years of the study, but they did significantly affect processing rates between acidic and circumneutral streams.The Unit is jointly sponsored by the National Biological Service, the West Virginian Division of Natural Resources, West Virginia University, and the Wildlife Management Institute.The Unit is jointly sponsored by the National Biological Service, the West Virginian Division of Natural Resources, West Virginia University, and the Wildlife Management Institute.     

Structural and functional measures of leaf-associated invertebrates and fungi as predictors of stream eutrophication

Eutrophication is a major threat to freshwater ecosystems worldwide that affects aquatic biota and compromises ecosystem functioning. In this study, we assessed the potential use of leaf decomposition and associated decomposer communities to predict stream eutrophication. Because leaf quality is expected to affect leaf decomposition, we used five leaf species, differing in their initial nitrogen concentration. Leaves of alder, chestnut, plane, oak and eucalyptus were placed in coarse-mesh bags and immersed in six streams along an eutrophication gradient to assess leaf decomposition and the structure of associated decomposer communities. A hump-shaped relationship was established between leaf decomposition and the eutrophication gradient for all leaf species, except for eucalyptus. Invertebrate biomass and density as well as fungal biomass and sporulation were lowest at the extremes of the gradient. Leaf-associated invertebrate and fungal assemblages were mainly structured by stream eutrophication. The percentage of shredders on leaves decreased, whereas the percentage of oligochaeta increased along the eutrophication gradient. The Iberian Biological Monitoring Working Party Index (IBMWP) applied to benthic invertebrates increased from oligotrophic to moderately eutrophic streams and then dropped sharply at highly and hypertrophic streams. Overall, leaf decomposition was a valuable tool to assess changes in stream water quality, and it allowed the discrimination of sites classified by the IBMWP within class I and class IV. Moreover, decomposition of most leaf species responded in a similar way to eutrophication when decomposition was normalized by the quality of leaves.     

Organic Matter Dynamics in a Tropical Gallery Forest in a Grassland Landscape

The high biodiversity of tropical forest streams depends on the strong input of organic matter, yet the leaf litter decomposition dynamics in these streams are not well understood. We assessed how seasonal litterfall affects leaf litter breakdown, density and biomass of aquatic invertebrates, and the microbial biomass and sporulation of aquatic hyphomycetes in a South American grassland ‘vereda’ landscape. Although litter production in the riparian area was low, leaf litter breakdown was high compared with other South American systems, with maximum values coinciding with the rainy season. Fungal biomass in decomposing leaves was high, but spore densities in water and sporulation rates were very low. Invertebrates were not abundant in litter bags, suggesting they play a minor role in leaf litter decomposition. Chironomids accounted for ~70 percent of all invertebrates; only 10 percent of non‐Chironomidae invertebrates were shredders. Therefore, fungi appear to be the drivers of leaf litter decomposition. Our results show that despite low productivity and relatively fast litter decomposition, organic matter accumulated in the stream and riparian area. This pattern was attributed to the wet/dry cycles in which leaves falling in the flat riparian zone remain undecomposed (during the dry period) and are massively transported to the riverbed (rainy season).     

Herbivory and growth in terrestrial and aquatic populations of amphibious stream plants

1. Many amphibious plant species grow in the transition between terrestrial and submerged vegetation in small lowland streams. We determined biomass development, leaf turnover rate and invertebrate herbivory during summer in terrestrial and aquatic populations of three amphibious species to evaluate advantages and disadvantages of aerial and submerged life.
2. Terrestrial populations had higher area shoot density, biomass and leaf production than aquatic populations, while leaf turnover rate and longevity were the same. Terrestrial populations experienced lower percentage grazing loss of leaf production (average 1.2–5.1%) than aquatic populations (2.9–17.3%), while the same plant dry mass was consumed per unit ground area.
3. Grazing loss increased linearly with leaf age apart from the youngest leaf stages. Grazing loss during the lifetime of leaves was therefore 2.4–3.1 times higher than mean apparent loss to standing leaves of all ages. The results imply that variation in density of grazers relative to plant production can account for differences in grazing impact between terrestrial and aquatic populations, and that fast leaf turnover keeps apparent grazing damage down.
4. We conclude that the ability of amphibious plants to grow submerged permits them to expand their niche and escape intense competition on land, but the stream does not provide a refugium against grazing and constrains plant production compared with the terrestrial habitat.     

Shredder–tadpole facilitation of leaf litter decomposition in a tropical stream

1. Leaf litter decomposition is one of the most important ecosystem processes in streams. Recent studies suggest that facilitation, in which litter is processed by a succession of species with differing abilities and requirements, may be important in making the nutrients bound in litter available to the stream assemblage.
2. We predicted that stream invertebrates that feed on terrestrial leaf litter (shredders) and tadpoles would facilitate leaf litter decomposition by changing the quality of leaf material directly via physical contact or indirectly via nutrient release. We experimentally examined the ability of shredders and tadpoles to break down leaves, independently and together, in artificial streams beside a natural forest stream.
3. The decomposition rate was greater when shredders and tadpoles were together than was expected from rates in single-species treatments, indicating that facilitation occurred. This facilitation operated in one direction only: the rate of leaf breakdown by tadpoles was higher when leaves had been partly processed by shredders, but there was no similar effect when leaves previously occupied by tadpoles were processed by shredders. We did not detect facilitation caused by indirect nutrient release.
4. Shredders may have benefited tadpoles by roughening leaf surfaces, making them easier for the tadpoles to consume and enhancing leaf breakdown in the presence of both taxa. This indicates that the loss of a single species can have impacts on ecosystem functioning that go beyond the loss of its direct contribution.     

Comparison of leaf processing rates under different temperature regimes in three headwater streams

1. We studied the effects of different temperature regimes on leaf litter processing in three forested Appalachian headwater streams of different pH (mean pH = 4.2, 6.5, 7.5).
2. We compared leaf breakdown rates, microbial biomass and macroinvertebrate shredder density and biomass between two 12-week processing periods (October–January and November–February) in each stream. Leaf processing rates were calculated both as k (day^–1) and k _d (degree day^–1).
3. There were no significant differences in processing rates ( k day^–1) between the two study periods for any leaf species in any stream. The average difference in temperature between the two study periods was 175 degree days. Shredder density was significantly higher during the earlier study period on 40% of the sample dates, but shredder biomass was not significantly different between the two study periods. ATP concentration was significantly higher during the early study period for 60% of the sample dates.
4. More significant differences in these variables (shredder density and biomass, ATP concentration) were seen among the three study streams than between the two study periods. This indicates that in this study other factors, particularly stream pH, contributed more to processing rate variation than did differences in thermal regime.     

Comparison of leaf processing rates under different temperature regimes in three headwater streams

1. We studied the effects of different temperature regimes on leaf litter processing in three forested Appalachian headwater streams of different pH (mean pH = 4.2, 6.5, 7.5).
2. We compared leaf breakdown rates, microbial biomass and macroinvertebrate shredder density and biomass between two 12-week processing periods (October–January and November–February) in each stream. Leaf processing rates were calculated both as k (day^–1) and k _d (degree day^–1).
3. There were no significant differences in processing rates ( k day^–1) between the two study periods for any leaf species in any stream. The average difference in temperature between the two study periods was 175 degree days. Shredder density was significantly higher during the earlier study period on 40% of the sample dates, but shredder biomass was not significantly different between the two study periods. ATP concentration was significantly higher during the early study period for 60% of the sample dates.
4. More significant differences in these variables (shredder density and biomass, ATP concentration) were seen among the three study streams than between the two study periods. This indicates that in this study other factors, particularly stream pH, contributed more to processing rate variation than did differences in thermal regime.     

Experimental dietary manipulations for determining the relative importance of allochthonous and autochthonous food resources in tropical streams

1. Autochthonous sources of organic matter appear to make a minor contribution to food webs in temperate forest streams, but their roles in supporting consumer biomass in tropical lotic environments have received little attention. We investigated the importance of autochthonous and allochthonous food sources to Brotia hainanensis (Pachychilidae), a detritivorous and algivorous snail common in Hong Kong hillstreams, using experimental dietary manipulations and assimilation-based analyses, including stoichiometry, carbon (C) and nitrogen (N) stable isotopes and fatty acid (FA) profiles.
2. Juvenile B. hainanensis collected in Pak Ngau Shek Stream were cultured under controlled laboratory conditions and fed for 2 months with either conditioned Liquidambar formosana (Hamamelidaceae) leaf litter or periphyton. Samples of B. hainanensis were also collected from the stream at the end of the experiment for comparison with snails reared in the laboratory.
3. Periphyton and leaf litter exhibited marked differences in C/N ratios, δ ¹³C and δ ¹⁵N values and FA profiles. Stable isotope analysis and FA profiling of laboratory-reared and field-collected B. hainanensis both confirmed that snails relied primarily on autochthonous foods, especially periphytic diatoms and cyanobacteria. Stoichiometry results indicated that periphyton was a more nutritious food (with lower C/N ratio) than leaf litter.
4. This is the first study demonstrating that the combined use of stable isotopes and FA profiles is an effective diagnostic tool to trace the basal food sources of consumers in natural stream habitats. Our findings further support the hypothesis that primary production in tropical streams is generally more important to aquatic consumers than inputs of terrestrial detritus.     

Decay rates of autumn and spring leaf litter in a stream and effects on growth of a detritivore

SUMMARY. 1. Differences in decay rates of autumn and spring balsam poplar (Populus balsamifera L.) leaf litter input to a stream and their effects on a lotic detritivore Tipula commiscibilis Diane were investigated.
2. Autumnal leaf litter decay rates were significantly greater than spring decay rates despite higher initial quality of spring leaves. Reduced spring/summer decomposition rates were the result of decreased microbial activity and biomass, and significantly lower numbers, kinds and biomass of macroinvertebrate detritivores.
3. Growth of the detritivore Tipula commiscibilis was significantly lower when fed spring leaves indicating that they were a poorer quality food source than autumn leaves.
4. Lower numbers of detritivores coupled with reduced leaf quality resulted in lower leaf litter decay rates characteristic of spring/summer.     

Antarctic stream ecosystems: physiological ecology of a blue-green algal epilithon

SUMMARY. 1. Several dozen summer meltwater streams are located in the McMurdo Sound region (c. 78°S 165°E) of southern Victoria Land. They are characterized by a highly variable flow regime at diel, seasonal and annual times caleis; wide fluctuations in temperature and nutrient content; and a very simple epilithic community of cyanophytes ( Nostoc spp., Oscillatoriaceae), bacteria, fungi and microherbivores.
2. The epilithon survives the dark Antarctic winter as dry, frozen mats which provide a large inoculum for growth the following summer. This overwintering assemblage retains a high metabolic capacity and responds rapidly to rehydration.
3. In a series of artificial substrate experiments, biomass accumulation rates were generally less than 0.1 In units d⁻¹. Colonization and growth on the substrates was inversely related to the suspended sediment load of the stream. There was also a visual correspondence between per cent algal cover of the natural streambed and the clarity of the streamwater. Sloughing losses may limit community biomass, particularly in the turbid flowing waters.
4. During running water conditions the mature communities had very low gross photosynthetic rates per unit chlorophyll (<0,1 μg C (μg chl a .h)⁻¹ and per unit carbon (<0,2 μg C (mg biomass C.h)⁻¹). Respiration was generally a high percentage (up to 92%) of gross photosynthesis, which probably reflected the high population densities of microheterotrophs in the community.
5. The floristically simple epilithic mats slowly accumulate to extreme biomass levels (>20 μg chl a cm ⁻², <20 mg C cm⁻²). Production rates per unit biomass are low, probably in response to the cold temperatures of the Antarctic stream environment, and the accumulated biomass represents several seasons of growth.