Abstract1. Many freshwater systems receive substantial inputs of terrestrial organic matter.Terrestrially derived dissolved organic carbon (t-DOC) inputs can modify light availability, the spatial distribution of primary production, heat, and oxygen in aquatic systems, as well as inorganic nutrient bioavailability. It is also well-established that some terrestrial inputs (such as invertebrates and fruits) provide highquality food resources for consumers in some systems.2. In small to moderate-sized streams, leaf litter inputs average approximately three times greater than the autochthonous production. Conversely, in oligo/mesotrophic lakes algal production is typically five times greater than the available flux of allochthonous basal resources.3. Terrestrial particulate organic carbon (t-POC) inputs to lakes and rivers are comprised of 80%-90% biochemically recalcitrant lignocellulose, which is highly resistant to enzymatic breakdown by animal consumers. Further, t-POC and heterotrophic bacteria lack essential biochemical compounds that are critical for rapid growth and reproduction in aquatic invertebrates and fishes. Several studies have directly shown that these resources have very low food quality for herbivorous zooplankton and benthic invertebrates. 4.Much of the nitrogen assimilated by stream consumers is probably of algal origin, even in systems where there appears to be a significant terrestrial carbon contribution. Amino acid stable isotope analyses for large river food webs indicate that most upper trophic level essential amino acids are derived from algae. Similarly, profiles of essential fatty acids in consumers show a strong dependence on the algal food resources.5. Primary production to respiration ratios are not a meaningful index to assess consumer allochthony because respiration represents an oxidised carbon flux that cannot be utilised by animal consumers. Rather, the relative importance of allochthonous subsidies for upper trophic level production should be addressed by considering the rates at which terrestrial and autochthonous resources are consumed and the growth efficiency supported by this food.6. Ultimately, the biochemical composition of a particular basal resource, and not just its quantity or origin, determines how readily this material is incorporated into upper trophic level consumers. Because of its highly favourable biochemical composition and greater availability, we conclude that microalgal production supports most animal production in freshwater ecosystems. | WHY DOES ALLOCHTHONY MATTER?A better understanding of where and how allochthony modifies aquatic food web processes will improve our ability to predict how land-use and climate change affect organic carbon export from watersheds to lakes and rivers and how this matter influences upper trophic level production in aquatic systems. If invertebrate and fish consumers in rivers and lakes are strongly subsidised by allochthonous carbon inputs, then watersheds and especially riparian zone management will potentially have as mu...

An experiment in >1000 river and riparian sites found spatial patterns and controls of carbon processing at the global scale.

Stable isotope studies of food webs in floodplains, large rivers, mangroves, and seagrasses have shown that, although a large proportion of the biomass may come from higher plants, microalgae provide a disproportionate amount of carbon assimilated by metazoan consumers. Evidence is building that this may also be the case for streams, especially those in the tropics. At the level of individual consumer species we also see that the apparent diet may not be reflected in the carbon assimilated. Tropical streams commonly have omnivoredetritivore species that potentially show this phenomenon. We tested these concepts in four moderately shaded sites in a stream in well-preserved Atlantic rainforest at Ilha Grande, Rio de Janeiro. We sampled aquatic insects, shrimps and fish as well as potential terrestrial and aquatic primary food sources. Carbon stocks from terrestrial sources predominated over carbon of algal origin (>99% of total). The primary sources of carbon showed distinctly different isotopic signatures: terrestrial sources had δ 13 C values close to −30‰, microalgae were −20‰ and macroalgae were −25‰. All fauna had δ 13 C values consistent with a carbon source derived from microalgae. Baetid mayflies and atyid shrimps exert a strong grazing pressure on periphyton and organic sediments but appear to assimilate predominantly microalgae. The palaemonid shrimp Macrobrachium olfersi also ingests large amounts of detritus of terrestrial origin, but apparently assimilates animal prey with algal δ 13 C signatures. These results support the growing view that tropical stream food chains are primarily algal based.

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Hutt Lagoon, 28" 1 I'S, 1 14" 15'E, 600 km north of Perth, Western Australia and lying 5 m below sea level is the site of a pilot plant erected by Roche Algal Biotechnology for growing and harvesting the alga Dunaliella salina. The lagoon is filled to a depth of 50-75 cm by rainfall (18% w/v salinity or above) during the winter months and is desiccated to a 5 cm or more thick crust during the summer. Salt from the crust used to prepare a growth medium for D. salina introduced a number of protozoa to the cultures, some of which made great inroads on the algal population. Most of the protozoa in the crust are presumed to be in the form of cysts and originate from more or less permanent seeps and pools (>5% w/v salinity) resulting from the inflow of water from the Indian Ocean on the west and from continental ground water on the east. The salt of the crust is thus a mixture of athalassic and thalassic origin. Only the lower reaches of the seeps are inundated by the winter water rise.Fourteen ciliates, 10 zooflagellates and 4 sarcodines were observed frequently enough in brines of over 15% (w/v) salinity to identify. At least one parasite of D. salina is included in the flagellate group. Although no concerted effort with the phytoflagellates was made, the rarely seen species D. peircei, D. jacobae and Ochromonas cosmopolitus were noted, as well as a Gymnodinium sp. The ciliates include the bacteriophagous Trachelocerca conifer, Metacystis truncata, Chilophrya utahensis, Rhopalophrya salina, Uronema marinum, Condylostoma sp. and Palmarella salina. Those eating both bacteria and algae were Nassula sp., Fabrea salina, Blepharisma halophila, Cladotricha sigmoidea, and Euplotes sp. Ciliates feeding on other ciliates include Podophrya sp. and Trematosoma bocqueti. Among the zooflagellates were several species of Monosiga, Rhynchomonas nasuta, Phyllomitus sp., Tetramitus salinus, T. cosmopolitus, Bodo caudatus, B. edax and 3 other distinctive Bodo species, one being parasitic on D. salina. All of the sarcodina fed on both algae and bacteria, except for the smallest amoeba (4 pm diameter rounded) which did not feed on algae, and included Heteramoeba sp. with both flagellate and amoeboid phases, an orange amoeba, an orange filopodforming organism and a colorless filopod-forming organism, the last three of unknown genus.The relationship of these protozoa to the lagoon and to D. salina culturing is discussed.

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1. Shrimps are abundant in many tropical coastal streams. Studies in Central America and the Caribbean have demonstrated the importance of shrimps in removing sediments and altering the composition of the benthos. Previous work in our study area showed that ephemeropterans and not shrimps were important in removing benthic material. 2. Here we used an experimental exclusion to test the hypothesis that shrimps exert strong influence on sediment dynamics with direct and indirect effects on the benthic algal and faunal community at a site where they are the predominant element of macrofauna. We used electricity to exclude Macrobrachium olfersi and Potimirim glabra from small quadrats (0.135 m 2 ) for 34 days in a stream located at Ilha Grande, Rio de Janeiro state, Brazil. We analysed benthic sediment dynamics and community colonisation during this period on artificial substrates in electrified and not-electrified quadrats. 3. Significantly higher sediment accrual had occurred in the electrified treatment after 14 days and persisted for the 34-day course of the experiment; substrates protected from shrimps contained about four times as much ash-free dry mass (AFDM) as those of the controls. After 34 days, significantly more pennate diatoms were present in the exclusion treatment, but chlorophyll was not significantly different between treatments. Densities of baetid ephemeropteran nymphs were significantly higher in the presence of shrimps. 4. We attribute all these effects to the atyid P. glabra, the more abundant and active shrimp observed in the control treatment. Our results suggest that atyid shrimps played an important role related to sediment removal on hard substrate by direct ingestion and 'bioturbation'. They reduced certain components of the periphyton (pennate diatoms) without affecting primary production (chlorophyll a). The activity of these shrimps on periphyton affected also faunal components such as baetid ephemeropterans and seems to determine the composition of the benthic community.

Coarse particulate organic matter is often broken down by specialist shredder invertebrates in temperate streams. In some tropical streams, larger, non-specialist, omnivorous fauna, (macroconsumers), particularly decapod shrimps and crabs, have been found to process coarse particulate matter. Larger shrimps and fish may also prey on or inhibit smaller invertebrates. Depending on the relative importance of larger and smaller fauna in leaf processing and in predatory interactions, we could expect that exclusion of larger fauna could either result in a decrease in leaf processing (if they were important in shredding or bioturbation) or increase in leaf processing if they negatively affected smaller shredders. We tested this by excluding fauna of different sizes from leaf peaks using bags with different sizes of mesh -0.2 mm (exclusion of most fauna), 2 mm (exclusion of larger fauna), and 10 mm (access to most fauna). Bag effect on leaf processing was minimized by constructing the bags of the same, fine, material, and sewing a relatively small window of the required mesh size. The experiment was conducted on two occasions in three streams of the urban forest of Parque Estadual da Pedra Branca, city of Rio de Janeiro. The three streams differed in larger fauna of shrimps (Macrobrachium potiuna), crabs, tadpoles, and fish. Packs were incubated for six time intervals and the rate of leaf processing calculated as the exponential rate of loss of leaf material. Rate of leaf processing was faster in bags with the largest mesh size; the rates in the other two mesh sizes were not statistically different. Rates varied between experiments and among streams. We could not attribute the faster leaf processing to any particular component of the larger fauna; the patterns of differences among streams and between experiments were not associated with particular taxa. There was no general trend of fewer smaller fauna in the presence of macroconsumers; the few smaller taxa that were different between mesh sizes were variously less and more abundant in the 10-mm mesh bags compared to the 2-mm. Known shredders were rare in the smaller fauna; the mining chironomid Stenochironomus was common, but was apparently not affected by larger fauna and apparently Handling editor: B. Oertli did not increase leaf processing. We conclude that macroconsumers and not smaller fauna had a positive effect on leaf processing, and this confirms a pattern observed in some other coastal Neotropical streams.