Bernd Spänhoff

Breakdown rates of wood in streams

Abstract The breakdown rates of natural branch wood were studied in 2 streams (lowland and upland), and the current knowledge on breakdown rates of wood in streams was reviewed. The breakdown coefficient of natural alder branches, k = 0.0650/y, in an unpolluted upland stream corresponded well to the few published data on breakdown rates of natural wood substrates in streams. Breakdown rates of Scots pine (Pinus sylvestris, k = 0.0698/y), bark-covered black alder (Alnus glutinosa, k = 0.0859/y), and debarked black alder branches (k = 0.0717/y) in a nutrient-rich sandy lowland stream indicated that breakdown rates differ among tree species. Data from the present and previous studies on the breakdown of natural wood and commercial wood products (wood veneer, tongue depressors, ice-cream sticks) showed that breakdown is slower for natural wood than for commercial wood products, in part because the commercial wood products used in the studies had high surface to volume ratios. A strong relationship exists between surface to volume ratio of the substrate and breakdown rates. The breakdown rates of wood in streams were associated with several abiotic factors, including the physical and chemical properties of the wood substrates used, dissolved nutrients, and water temperature. Overall, the review of published breakdown rates emphasized that the current knowledge on the breakdown rates of wood in streams, especially the temporal patterns of microbial activities, is very limited, and heterogeneous experimental designs in previous studies prevent general conclusions. Nevertheless, measuring wood breakdown rates could be a promising tool for practical application in assessment of the ecological integrity of streams.

Simple method for rating the decay stages of submerged woody debris

Previous investigations of interactions between aquatic invertebrates and decomposing wood substrates used wood decay classification systems based on external characteristics such as texture, shape, and color. These methods may be inaccurate for the assessment of wood decay because external appearance does not necessarily represent the physical state of decomposing wood. We tested an alternative method based on relative wood density adapted from terrestrial wood decomposition studies. We compared rating the physical condition of submerged twigs and branches by the loss of relative density to the traditional method based on external characteristics. Rating wood decay using loss of relative density showed distinct advantages compared to estimates by external characteristics. Measuring density delivered reproducible data independent of wood size or investigator bias. There was a correlation between decreases in relative wood density and increasing water content. In contrast, maximum water content was not correlated with the decay class rated by external characteristics. In addition, decay rating using external characteristics was influenced by the size of the woody debris. Rating wood decay using relative density is a simple and useful tool for characterizing woody debris in different stages of decay.

Epixylic biofilm and invertebrate colonization on submerged pine branches in a regulated lowland stream

Epixylic biofilm and invertebrate assemblages on pine branches (Pinus syl- vestris) experimentally submerged in October 2000 were investigated, after an initial colonization period of 3 months, by two-monthly sampling intervals for 13 months in a nutrient-rich sandy lowland stream subjected to flow disturbances caused by infre- quent impoundment openings. After 12 weeks of exposure an epixylic biofilm with 0.58 ± 0.25 µg/cm 2 chlorophyll-a (mean ± 1 SE) and 0.23 ± 0.04 µg/cm 2 ATP, as well as an abundant invertebrate community (14326 ± 2532 Ind/m 2 ; biomass: 974.6 ± 360.1 mg/m 2 ) became established on the branch surfaces. During the subsequent sam- pling dates invertebrate numbers decreased significantly during periods of high dis- charge and simultaneously chlorophyll-a values of epixylic biofilms increased, likely due to reduced feeding by invertebrate grazers. During periods with low discharge fluctuations and mainly low flow conditions, the epixylic biofilm, especially algal growth, was negatively correlated with invertebrate grazer and shredder numbers. After the initial growth period of 12 weeks, ATP values of the epixylic biofilm showed a strongly negative response to sand deposited on the wood surfaces. The present study displays the influence of seasonal discharge fluctuations and sand deposition on the wood surfaces on epixylic biofilms and invertebrate assembla- ges on experimentally submerged pine branches, but also indicate interactions between food sources (algae and fine particulate organic matter) and invertebrates (grazers and collector/gatherers).

Ingestion and utilization of wood by the larvae of two Trichoptera species, Lasiocephala basalis (Lepidostomatidae) and Lype phaeopa (Psychomyiidae)

Food-intake and digestion of Lasiocephala basalis and Lype phaeopa, both strongly associated with submerged wood, were investigated and compared. Gut-content composition of both taxa was analysed and their capability of digesting cellulose and fungal hyphae was measured by enzyme assays. Gut contents of Lype phaeopa consisted of a significantly higher proportion of wood (88.5 % ± 5.8 SD) than those of Lasiocephala basalis (77.8 % ± 8.2). The amount of amorphous detritus in the guts was significantly higher in Lasiocephala basalis (16.6 % ± 6.9) compared to Lype phaeopa (7.5 % ± 3.7). Algae, inorganic material and fungal hyphae were found in comparatively small amounts. Invertebrate remains were detected only in Lasiocephala basalis. To determine the activity of two cellulolytic enzymes, β-D-glucoside glucohydrolase and 1,4-β-D-glucan cellobiohydrolase, substrate-analogues covalently bound to the fluorochrome methylumbelliferyl were used. Cellulose digestion by means of the four mechanisms described by MARTIN (1991) can be precluded, as 1,4-β-D-glucan cellobiohydrolase was not traceable. Activity of chitobiase could be detected in both species. Therefore, chitin of fungal hyphae might be a potential food resource. Gluco-hydrolase and chitobiase showed a significantly lower turnover rate with a higher substrate affinity in Lype phaeopa than in Lasiocephala basalis. Different life patterns of the two species are discussed as a reason for these differences. Despite lacking a complete cellulase system in their guts, both taxa may be capable of utilizing cellulose preconditioned by microorganisms. This is supported by results of the gut-content analyses combined with conclusions drawn in a previous study (SCHULTE et al. 2002).

Do grazing caddisfly larvae of Melampophylax mucoreus (Limnephilidae) use their antennae for olfactory food detection

Abstract.u2002 The behaviour of larvae of Melampophylax mucoreus (Limnephilidae; Trichoptera) is studied to test their capacity for food detection over long distances by olfaction. In aquaria experiments with stagnant water, the larvae rapidly accumulate on a biofilm‐covered stone (offered food patch) but not on control stones without biofilm‐cover. Larvae in an artificial flow channel (flume), with uni‐directional flow between an arrangement of stones covered with biofilm and stones without biofilm, can find food patches upstream from their location more rapidly than downstream. Stimuli transported towards the larvae by the water current might assist food location. However, no significant differences could be found in the movement of larvae with intact antennae compared with larvae without antennae. The results lead to the conclusion that the larvae of M. mucoreus find food patches mainly by random movement, which is assisted by their high mobility. Long‐distance olfactory orientation towards food patches, mediated by antennae, can be precluded. Gustatory stimuli appear to play an important role in contact‐chemoreception to identify biofilm patches on stone surfaces. Scanning electron microscopy studies of the larval head reveal that the antennae of M. mucoreus are small pegs (50u2003µm in length) with an apical plate (15u2003µm in diameter). The mouthparts of the larvae, especially the maxillary palps, show a variety of different sensilla types that are possibly used for contact‐chemoreception. These sensilla also could be responsible for the orientation towards a food resource upstream of the larvae.

Laboratory investigation on community composition, emergence patterns and biomass of wood-inhabiting Chironomidae (Diptera) from a sandy lowland stream in Central Europe (Germany)

Wood-inhabiting chironomid communities were investigated from June 1998 to July 1999 by laboratory rearing of randomly collected submerged branches from a headwater and mid-reach site of a sandy lowland stream, separated by an impoundment used for fishery purposes. Total annual emergence (males and females) from headwater samples was higher (2551 ind m–2 y–1) compared to the mid-reach (1576 ind m–2 y–1), which could be due to disturbances caused by frequent impoundment openings resulting in high discharge events. Chironomid community from branches comprised three subfamilies, with Orthocladiinae (18 species, 2189 ind m–2 y–1) clearly predominating at the headwater (total of 36 species). Mid-reach samples (total of 48 species) showed similar emergent numbers of Orthocladiinae (19 species, 786 ind m–2 y–1) and Chironominae (26 species, 764 ind m–2 y–1). Tanypodinae were caught very rarely in the laboratory emergence (12 ind m–2 y–1 at both sites). Shannon-Wiener diversity index for the mid-reach chironomid community was higher (2.52) than for the headwater community (1.68). Chironomid species composition on woody debris was similar between stream sites, with a Sørensen index of 0.75, but showed different dominance structures indicated by a Wainstein index of 0.26. Total adult biomass (in the manner of dry mass) during the study period was higher for wood-dwelling chironomids from the headwater (158.2 mg m–2 y–1) compared to the mid-reach (123.8 mg m–2 y–1), but individual biomass was higher for mid-reach chironomids (0.079 mg ind–1 vs. 0.062 mg ind–1 at the headwater), indicating the predominance of larger species. Total biomass of wood-inhabiting chironomids in the investigated lowland stream was low compared to other field emergence studies, which could be attributed to the laboratory approach for investigating the emergence from a single substrate type (submerged wood) instead of the integrative field surveys where chironomids from all habitats were caught. Main reason for the lack of chironomid species closely associated to wood in this sandy lowland stream could be infrequent but episodic disurbances caused by the anthropogenic induced highly fluctuating discharge regime of the downstream study site.

Seasonal decline of body mass in adults of the wood-dwelling caddisfly Lype phaeopa (Psychomyiidae)

Larvae of the psychomyiid caddisfly Lype phaeopa (Stephens 1836) were reared under close to outdoor temperature conditions to investigate the seasonal fluc- tuations in body masses of emergent adults. The adult emergence period lasted from late February until late September, with two emergence peaks of males (March and August) and one for females (September). Females were always larger than males, illustrating a strong sexual size dimorphism. Decreasing body mass of emergent adults coincided with increasing temperature during the seasons until the end of the emer- gence period. Both sexes showed a significant relationship between body mass and wing length, which allowed me to estimate the loss of female body mass due to oviposition (55.7% ± 4.9). Egg size remained constant throughout the study (ovoid shape, mean length 216 µm, width 186 µm) but number of eggs decreased. The results imply that increasing temperature could be responsible for the decline in size and body mass of emergent adults (both sexes). A reason for this size reduction could be a constant growth rate (biomass increment) but an accelerated developmental rate induced by increasing tem- perature.

Ecological aspects of the external morphology and functionality of the psychomyiid female ovipositor (Insecta, Trichoptera)

Females of most psychomyiid species bear an elongated ovipositor enabling them to oviposit their eggs into grooves and cavities in different kinds of substrates in freshwaters. Although the preference of psychomyiid species for oviposition into distinct substrates and the subsequent larval life performance in aquatic habitats is already known, the mode of oviposition and the functional morphology of the elongated ovipositor have not been described until now. In this study we present SEM photographs of some psychomyiid species with female ovipositors of different lengths and shapes, explaining their preference for oviposition into distinct substrates. Additionally, we discuss some ecological aspects of the ovipositor shapes and the mode of egg laying for psychomyiids. The female ovipositor consists of two body segments, with an elongated IXth and a distinctly shorter Xth segment, which is flexible and can be pushed up dorsally when releasing the eggs. On the basis of SEM photographs, we suppose that the opening of the channel through which the eggs were released is located ventrally near the ovipositor tip. The ovipositor itself is characterised by a ventral cleft reaching from the gonoporus of segmentxa0IX to the tip of the last segmentxa0X. We stored adult females abdomina overnight in different aqueous dilutions of ethanol leading to different stages of swelling of the ovipositors (grade of swelling in distilled water >30% ethanol >70% ethanol). Some internal membrane-like structures, normally infolded into the ovipositor, became visible by the swelling of the ovipositor. We discuss the possible rearrangement of the organisation of segment IX resulting in the development of an ovipositor on the basis of SEM photographs of the differently swollen ovipositors.

Effect of permanent shading on adult size and biomass of the caddisfly Lype phaeopa (Stephens, 1836) (Psychomyiidae)

To investigate the effect of shading on the development of aquatic insect larvae, estimated by the size and biomass of freshly emerged adults, a laboratory experiment was conducted in which larvae of the psychomyiid caddisfly Lypephaeopa were reared in glass aquaria covered by gauze pervious to light and compared to aquaria which were covered by a black textile distinctly reducing the light irradiation. Seven submerged branches colonized by larvae of L.phaeopa were collected from a small lowland stream, cut in two equally sized halves and one of the halves was reared under natural light and the other one under shaded condition. The hypothesis of the study was that larvae reared under natural light conditions can develop faster resulting in earlier emergence and larger adults compared to larvae reared under a shaded conditions. Females were always significantly larger and heavier than simultaneously emergent males (paired t-test, p < 0.001) independent of the treatment. The experiment revealed no effects of the shading on the adult size and biomass of emergent adults, neither for males (t-test, p = 0.197) nor for females (p = 0.303). Adult biomass was influenced by seasonal temperature (smaller adults during the warm summer months). Females’ biomass was also different between the branches independent from the treatment (one way ANOVA, p = 0.002) whereas males showed only a trend (p = 0.055). The shading displayed a significant effect on the emergence date of males with earlier emergence in the light treatment compared to the dark treatment (t-test, p = 0.046), but this could be not ascertained for females (p = 0.828). The experiment indicated that shading has no significant effect on the adult biomass of L.␣phaeopa, but it could delay the time of emergence in males. Main driving factors of larval development and subsequent adult biomass in L. phaeopa were temperature and habitat quality.

The effect of food supply and larval cannibalism on adult size and biomass of the grazing caddis-fly Melampophylax mucoreus (Limnephilidae)

Abstract.u2002 1. Larvae of the grazing caddis‐fly Melampophylax mucoreus were reared in a laboratory experiment investigating the effect of food availability on different substrates and cannibalism on the size and biomass of emergent adults. All experiments were performed in stream‐water filled, aerated aquaria under controlled temperature and light conditions. Larvae (fourth and fifth instar) were reared in aquaria (50 larvae in each) with three substrate scenarios: (i) limestone (LS), (ii) limestone and leaf litter (LSu2003+u2003L), and (iii) silicate stone (SS).