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, d 13 C, and d 15 N 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 13 C and 15 N 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.

Relationships of shredders, leaf processing and organic matter along a canopy cover gradient in tropical streams

“…In JAR, despite high canopy cover, local fauna is highly supported by autochthonous carbon (Neres-Lima et al, 2016). These observations match with the prediction that even highly covered tropical streams have enough light incidence to sustain a reasonable level of primary produc- (Bunn et al, 1999a(Bunn et al, , 1999bLau et al, 2009). In this manner, we speculate that shredders may be consuming some autochthonous carbon along with their predominant diet of leaves and that they might benefit from algae attached to leaves (Guo et al, 2016).…”

“…Evidence from global studies indicates high variability in shredder occurrence and in breakdown rates across tropical sites (Boyero et al, 2012(Boyero et al, , 2015, which corroborates contrasting results obtained in tropical regions. A growing body of evidence emphasizes the importance of algal carbon in tropical food webs of forested small streams (Salas and Dudgeon, 2001;Mantel et al, 2004;Brito et al, 2006;Li and Dudgeon, 2008;Lau et al, 2009;Neres-Lima et al, 2016;Brett et al, 2017) and it has been claimed that the contribution of macroinvertebrate shredders to leaf breakdown is small in certain tropical running waters due their scarcity (Dudgeon and Wu, 1999;Dobson et al, 2002;Gonçalves et al, 2006bGonçalves et al, , 2007Ardón and Pringle, 2008;Alvim et al, 2015). On the other hand, there is contrary evidence of high diversity and abundance of macroinvertebrate shredders, high secondary production and importance on leaf breakdown (Cheshire et al, 2005;Camacho et al, 2009;Yule et al, 2009;Encalada et al, 2010;Masese et al, 2014;Tonin et al, 2014;Andrade et al, 2017;Neres-Lima et al, 2017).…”

Preservation effects on C/N ratios and stable isotope signatures of freshwater fishes and benthic macroinvertebrates

“…N signatures and molar C/N ratio of conspecifics-including the species involved in this studyshow a degree of inter-stream variation (Lau et al 2009a,b), we collected animals from different streams in order to incorporate this natural variation in our investigation of preservation effects. As far as we are aware, such intraspecific variation has not been considered in previous studies of this type.…”

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“…In contrast, longer preservation of C. cantonensis in ethanol led to greater variability in d 13 C shifts. Brotia hainanensis has a lower lipid content than either of these shrimps (Lau et al 2009a), and thus the effects of lipid hydrolysis by ethanol would have been relatively minor. Previous studies have also recorded interspecific variation in ethanol-induced d 13 C shifts; however, they are less predictable than those caused by formalin (Sarakinos et al 2002;Syväranta et al 2008), perhaps because interspecific differences in lipid content influence the degree of lipid hydrolysis by ethanol (Von Endt 1994).…”

“…Given that d 13 C and d 15 N signatures and molar C/N ratio of conspecifics-including the species involved in this studyshow a degree of inter-stream variation (Lau et al 2009a,b), we collected animals from different streams in order to incorporate this natural variation in our investigation of preservation effects. As far as we are aware, such intraspecific variation has not been considered in previous studies of this type.…”