Mineral grains in caddisfly pupal cases and streambed sediments: Resource use and its limitation through conflicting resource requirements

Abstract

Sand and fine gravel in streambed sediments are considered to be an overabundant resource for caddisflies that build cases from it. However, larvae of Rhyacophila and Hydropsyche build pupal cases with material collected near the pupation site and typically attach these cases to cobbles in riffles, where the rapid flow facilitates respiration but decreases the availability of case-building material through erosion. Analyzing mineral grain sizes of the pupal cases and the streambed in samples taken from cobbles in a stream riffle, we found that the overall mass use in pupal cases of Rhyacophila and particularly Hydropsyche significantly increased with local mass availability of building material, indicating that mineral grains can be a limited resource for these taxa. In addition, the most abundant species (Hydropsyche siltalai Döhler) significantly changed the case architecture if the preferred grain fraction (2.5-3.15 mm) was a limited resource. Under resource limitation of the preferred fraction, H. siltalai used the 1.6-2-mm fraction instead, which could reduce the resistance of the cases to damage resulting from floods that move coarser bottom material. Our findings suggest that, in streams or near shores of lakes and oceans, water currents that modify grain availability can create conflicts in resource requirements for invertebrates, particularly if they need locally available fine grains for the building, coarse grains for attachment, and high oxygen renewal rates for metabolic needs.There can be little doubt that resource limitations play a key role in the understanding of the biology and ecology of living systems. Therefore, resource limitations figure prominently in studies that address the potential role of intra-or interspecific competition for resources, which in turn determine niche dimensions, community composition, and diversity (e.g., Begon et al. 1986). Corresponding to the importance of the topic, recent research in limnology and oceanography focused on the question of how seemingly overabundant resources can be limited because subtle constraints interfere with their use. For example, dissolved inorganic carbon (C) in seawater is so abundant, but the discovery of carboxylating enzyme activities suggests that inorganic C may be limiting for marine algae (Buitenhuis et al. 2003;Rost et al. 2003). Likewise, there is growing evidence that the usability of overabundant organic C for aquatic bacteria, zooplankton, and fish depends on size, morphology, mineral composition, toxicity, and/or other biochemical features of the C source (Becker and Boersma 2003;Castillo et al. 2003;Holzman and Genin 2003;Strom et al. 2003). This paper contributes to the research on the subtleties in the use of overabundant resources in aquatic 1 Corresponding author (statzner@biomserv.univ-lyon1.fr). AcknowledgmentsWe thank Marie-Francoise Arens, Sylvain Dolédec, and JeanMichel Olivier for help in the field, and particularly Marie-Noelle Truchet, who helped to sort the samples, fractioned them, and accurately dete...

Mineral grains in caddisfly pupal cases and streambed sediments: assessing resource use and its limitation across various river types

“…From these, only the August samples contained sufficient identifiable pupal cases to be included here. Statzner et al (2005) reported temporal stability of overall grain availability and use (for RCWs) across these three dates (confirmed here for overall grain use by ICWs through a Kruskal-Wallis tests on differences among dates (P = 0.450)), so I pooled these data to obtain a sample size of 129 quantitative samples (one sample was lost because of a mishap). At the other sites, 50 quantitative samples each were obtained at one occasion on 1-2 days during the peak pupal season (Table 1; note that one sample from the Bez was also lost).…”

“…Expanding previous work on caddisfly pupal cases from the Furan River (Statzner et al, 2005), I re-use published data on RCWs from that study that contributed to tests of the hypotheses of the present study (avoiding repetition about taxon specific grain resource limitations). In addition, I use unpublished data on ICWs from that previous study and on RCWs and ICWs from four other rivers of varying size that are running through regions without or with different well-known wines (Johnson and Duijker, 1993), which is a clear sign for differences in soil and climate among them.…”

“…From a preliminary study described in Statzner et al (2005), I knew that the intraspecific variability of the details in pupal case architectures of RCWs at similar habitat conditions can be relatively high and that it requires many specimens per species and many samples to discover effects of grain availability on grain use. Therefore, I stratified the riffles in the study sites and randomly sampled cobble (y 6-20 cm) substrates with a relatively high shear stress (see Table 1), as these conditions are typically used by many RCWs and ICWs for pupation.…”

“…I calculated building material availability as described by Statzner et al (2005), ignoring the size <0.125 mm (because the mesh size of the field net was 0.1 mm) and using all the remaining sediments of a sample, plus the fragments of the empty pupal cases, plus half of the material used in the pupal cases of the RCWs. The rationale to include half of the material used in RCW cases in this calculation related to the observation that the pupal cases in a sample could contain all development stages (prepupa to mature pupa).…”

“…Studying this idea at one riffle site, Statzner et al (2005) found that the overall mass use in pupal cases of Rhyacophila and particularly Hydropsyche significantly increased with local mass availability of building material, indicating that mineral grains can be a limited resource for these RCW taxa. In addition, the most abundant species (H. siltalai Do¨hler) significantly changed the case architecture if the preferred grain fraction (2.5-3.15 mm) was a locally limited resource.…”