Danny C. P. Lau

Are autochthonous foods more important than allochthonous resources to benthic consumers in tropical headwater streams

Abstract Increasing evidence suggests that autochthonous foods are the principal basis of consumer production in tropical forest streams, despite the predominance of terrestrial detritus inputs. The relative importance of autochthonous and allochthonous energy for the dominant benthic consumers was investigated in 3 tropical headwater streams with different shading conditions in Hong Kong with a combination of assimilation-based analyses: stoichiometry, C and N stable isotopes, and fatty acid (FA) profiling. The snail Brotia hainanensis (Pachychilidae), shrimps Caridina cantonensis (Atyidae) and Macrobrachium hainanense (Palaemonidae), and their potential basal food sources (leaf litter, fine particulate organic matter [FPOM], periphyton, cyanobacteria, and filamentous algae) were collected in Tai Po Kau Forest Stream (shaded 1), Shing Mun Stream (shaded 2), and Pak Ngau Shek Stream (open) during the 2004 dry season (January and February). All samples were analyzed for C:N ratios, δ13C, and δ15N values. Total FAs were extracted from each sample, and concentrations of 35 important FAs were analyzed by gas chromatography–mass spectrometry (GC–MS). C:N ratios of algal foods were markedly lower than those of terrestrial detritus and similar to those of the test animals at all 3 sites, a result that suggested that autochthonous sources were relatively more nutritious than were allochthonous sources. Autochthonous foods were more 13C and 15N enriched than were allochthonous foods at all sites. The algal sources contributed to 29 to 98% of consumer biomass, generally more than was attributed to the terrestrial sources (2–71%). Consumers also showed distinctive FA profiles indicating consumption of autochthonous foods, especially periphytic diatoms and cyanobacteria, as revealed by the elevated concentrations of FA biomarkers such as palmitoleic (16:1[cis-9]) and eicosapentaenoic acids (20:5[all cis-5,8,11,14,17]) in the consumers, periphyton, and cyanobacteria. Our results suggest that autochthonous resources are possibly more important than allochthonous foods to secondary production in tropical headwater streams.

Autochthonous resources are the main driver of consumer production in dystrophic boreal lakes

Dystrophic lakes are widespread in temperate regions and intimately interact with surrounding terrestrial ecosystems in energy and nutrient dynamics, yet the relative importance of autochthonous and allochthonous resources to consumer production in dystrophic lakes remains controversial. We argue that allochthonous organic matter quantitatively dominates over photosynthetic autotrophs in dystrophic lakes, but that autotrophs are higher in diet quality and more important for consumers as they contain essential polyunsaturated fatty acids (PUFA). In a field study, we tested the hypotheses that (1) autochthonous primary production is the main driver for consumer production, despite being limited by light availability and low nutrient supplies, and greater supply of allochthonous carbon, (2) the relative contribution of autotrophs to consumers is directly related to their tissue PUFA concentrations, and (3) methane-oxidizing bacteria (MOB) provide an energy alternative for consumers. Pelagic and benthic consumer taxa representing different trophic levels were sampled from five dystrophic lakes: isopod Asellus aquaticus, megalopteran Sialis lutaria, dipteran Chaoborus flavicans, and perch Perca fluviatilis. Based on carbon and nitrogen stable isotopes, the relative contributions of autochthonous (biofilms and seston) and allochthonous (coarse particulate and dissolved organic matter) resources and MOB to these taxa were 47-79%, 9-44% and 7-12% respectively. Results from fatty acid (FA) analyses show that the relative omega3-FA and PUFA concentrations increased with trophic level (Asellus < Sialis and Chaoborus < Perca). Also, eicosapentaenoic-acid (EPA), omega3-FA and PUFA concentrations increased with the autochthonous contribution in consumers, i.e., a 47-79% biofilm and/or seston diet resulted in tissue EPA of 4.2-18.4, omega3 FAs of 11.6-37.0 and PUFA of 21.6-61.0 mg/g dry mass. The results indicate that consumers in dystrophic lakes predominantly rely on energy from autotrophs and that their PUFA concentrations are dependent on the relative contribution of these autochthonous resources. The limited energy support from MOB suggests they are not negligible and are potentially an integral part of the food webs. Our findings show that autochthonous resources are the main driver of secondary production even in dystrophic lakes and offer new insights into the functioning of these ecosystems.

Developing a circumpolar monitoring framework for Arctic freshwater biodiversity

Arctic freshwater ecosystems are facing unique challenges through the interaction of natural and human-induced stressors such as climate change and industrial development. Much is unknown about the biodiversity of Arctic freshwaters, although it is believed to have already been affected by climate change. A pan-Arctic monitoring strategy is critically needed to improve abilities to detect and understand ongoing and future changes in Arctic freshwater ecosystems. The challenging issues that Arctic freshwater monitoring must address include: the large diversity of Arctic freshwater ecosystems, varying levels of stressor impacts across the Arctic, lack of historical baseline research and monitoring coordination, and poor among-country standardization of sampling protocols. In response, the Arctic Councils Freshwater Expert Monitoring Group of the Circumpolar Biodiversity Monitoring Program (Conservation of Arctic Flora and Fauna) is developing a framework for monitoring Arctic freshwater biodiversity that will lead to regular reviews of the state of freshwater ecosystems across the circumpolar Arctic. The parameters of primary focus for the monitoring framework are classified by focal ecosystem components (FECs), which are biotic or abiotic factors that are ecologically pivotal, charismatic and/or sensitive to changes in biodiversity. FECs are placed in the context of expected ecosystem change through the development of testable impact hypotheses (or predictions) that outline a cause-effect framework regarding how change in environmental and anthropogenic stressors is expected to affect FECs. These prediction statements provide both guidelines for future scientific data collection and a focus for management decision-making. Here we discuss the design of a proposed monitoring framework and the development of impact hypotheses that focus on climate change effects. We emphasise the connectivity between science, monitoring and management necessary to implement the framework across the Arctic.

Experimental dietary manipulations and concurrent use of assimilation-based analyses for elucidation of consumer–resource relationships in tropical streams

The relative contribution of autochthonous foods to consumer biomass in small tropical streams is unknown, but extrapolation of findings from temperate forest streams, where food webs are based on allochthonous resources, might be misleading. Experimental dietary manipulations were conducted to investigate the food used by the snail Brotia hainanensis (Pachychilidae), a generalist primary consumer common in Hong Kong streams, through the concurrent use of stoichiometry, stable isotope analysis (SIA) and fatty acid (FA) profiling. Juvenile B. hainanensis collected from the field were cultured under laboratory conditions and fed with conditioned leaf litter, periphyton or commercial fish-food flakes for 6 months and then compared with field-collected snails at the end of the trial. The results of the SIA and FA profiling showed that snails depended primarily on algal food. Prolonged feeding with leaf litter put B. hainanensis under elemental constraints and litter-fed snails deviated from strict stoichiometric homeostasis. Periphyton-fed, flake-fed and field-collected snails contained more total lipids and autochthonous FA biomarkers than litter-fed snails. The concurrent application of assimilation-based analyses allowed effective and accurate elucidation of consumer-resource relationships and, in the present study, confirmed the importance of autochthonous energy in a tropical stream food web. This approach will be useful for investigating complex trophic interactions.