I AQUATIC MICROBIAL ECOLOGY I Vol. 11: 21-29, 1996 Published August 29 Aquat Microb Ecol I i I Seasonal and spatial variability of planktonic heliozoa in Lake Constance Uwe Zimmermann*, Helga Miiller**,Thomas Weisse*** Lirnnological Institute, University of Konstanz, PO Box 5560, D-78434 Konstanz, Germany ABSTRACT: Planktonic heliozoa were investigated at a mid-lake and an inshore station in Lake Constance (Germany)from April to November 1993. Integrated water samples were taken over 0 to 8 m and 8 to 20 m depth intervals at the deep mid-lake station and over 0 to 2 m depth at the shallow inshore station. Heliozoans were counted and identified to genus level in live samples. The following genera were identified: Actinophrys, Raphidocystis, Heterophrys, Chlarnydaster, Choanocystis, Raphidiophrys, and Pterocystis. Small heliozoans (10 to 20 pm, mainly Heterophrys and Choanocystis) generally dominated the con~n~unityin terms of abundance. Large genera (Actinophrys, Raphidocystis) were, however, the major contributors to total biovolume. Total cell concentrations remained below detection limits from April to mid-June. Maxima of up to 6.6 ind. ml-' were observed in summer; smaller peaks occurred in autumn. Heliozoan cell numbers were significantly positively correlated wth chlorophyll a concentration close to the surface. Negative trends were found in relation to potential heliozoan competitors or predators such as rotifers and crustacea. Community biovolumes of up to 60 mm3 m-3 were recorded in mid-summer The seasonal succession of the dominant genera was sirni- lar at both stations. The vertical distribution of heliozoans, examined on 2 occasions in summer and autumn, was positively correlated with chlorophyll a and temperature. We further studied the horizon- tal distribution of heliozoans at 8 stations across the northwestern part of Lake Constance on one occa- sion in late summer. Cell numbers recorded varied by a factor of 2. In situ growth rates of heliozoans were measured in diffusion chambers after the exclusion of larger potential predators at the mid-lake station on 2 occasions. Growth rates of distinct genera ranged from 0.06 to 0.59 d-l, community growth rates from 0 3 to 0.5 d.' Based on growth rates and biomasses, we calculated the potential heliozoan production Results suggest that during periods of their maximum abundance heliozoan community production is similar to average production estimates of ciliates and heterotrophic nanoflagellates. The seasonal mean heliozoan production is, however, equ~valentto only about 1 % of the combined ciliate and flagellate production. KEY MIORDS: Heliozoa Planktonic protozoa . Seasonal dynamics - Lake Constance INTRODUCTION recently, Rainer (1968) described the preference and tolerance of 22 species in relation to environmental Heliozoans have been known for more than 2 cen- factors such as pH and temperature. Takahashi & Yi turies; yet they have rarely been included in limnolog- Ling (1980) examined the vertical distribution of Sti- ical studies. The first detailed account on the ecology cholonche zanclea in the Pacific Ocean. BerzinS & of these sarcodines was given by Penard (1904).More Stensdotter (1990) investigated the adequacy of differ- ent substrate types for rhizopods and some heliozoans. Until recently, the prevailing opinion was that helio- 'Present address: Labor fiir hydraulisches Versuchswesen. zoans prefer solid substrate surfaces like sediment and Cewasserschutz und Okologie (Hydrolabor Schleusin- macrophytes (cf. Rainer 1968) or live attached to detri- gen), PO Box 66, D-98549 Schleusingen, Germany tal particles (Laybourn-Parry et al. 1991). Research "Addressee for correspondence. E-mail: [email protected] conducted over the last 2 decades has revealed, how- "'Present address: Max Planck Institute for Limnology, PO ever, that heliozoans are also common in freshwater Box 165, D-24302 Plon. Germany plankton, at times even in high numbers comparable to 0 Inter-Research 1996 Aquat Microb Ecol 11. 21-29, 1996 those of other pelagic protozoa (reviewed by Arndt between 100 and 300 m1 of the original sample were 1993). examined in a settling chamber (2 m1 vol) by inverted In the large, prealpine Lake Constance, the micro- microscopy using phase contrast. We checked the pre- bial food web has been examined for more than 10 yr. cision and accuracy of this melliucl by splitting 1 sam- Among protozoa, these studies have concentrated on ple into 3 subsamples and counting each subsample 3 the ecology of heterotrophic nanoflagellates (Gude times. The coefficient of variability (CV) of total cell 1986, Weisse 1990, 1991, Jiirgens & Giide 1991) and numbers calculated according to Lozan (1992) was 7 "h ciliates (Muller 1989, Muller et al. 1991, Muller & at abundances ranging from 500 to 1000 heliozoans 1-'. Weisse 1994). In this paper, we extend the existing The CV of the abundance of individual genera varied knowledge on the significance of pelagic protozoa by from 8.5% in the most numerous up to 86.5% in the presenting the first quantitative data on heliozoa in rarest genus. Lake Constance. Identification to the genus level followed the keys by Rainer (1968) and Page & Siemensma (1991). Live specimens were examined by phase contrast micro- MATERIALS AND METHODS scopy for different morphological criteria, especially size and shape of skeletal elements. Cell size was mea- Lake Constance ('Bodensee') is a warm-monomictic sured by a calibrated eyepiece micrometer. Biovolume prealpine lake with a surface area of 540 km2 and a was calculated assuming spherical cell bodies of the maximum depth of 252 m (Geller & Gude 1989). The heliozoans. lake is thermally stratified from April through October. Parallel counts of total heliozoans, without discrimi- Its current trophic state is considered mesotrophic nation of genera, were performed in fixed samples for (Tilzer et al. 1991). Our study was conducted in Uber- the entire series at the mid-lake station, depth interval linger See, a fjord-like basin with an area of 65 km2 0 to 8 m. An aliquot of 250 m1 was fixed with acid Lu- and maximum depth of 147 m, in the northwestern part gol's iodine (final concentration 1 %), allowed to sedi- of the lake (Fig. 1). ment for 24 h and then concentrated to 100 m1 by gently Water samples were taken with a 2 m long water bot- removing the supernatant. Half of the remaining 100 m1 tle at weekly intervals from April to November 1993. was counted by inverted microscopy (Utermohl 1958). Samples were integrated for 0 to 2 m depth at a littoral Generally, heliozoan numbers obtained from live station (M2, z = 2.2 m) and for 0 to 8 m and 8 to 20 m counts were slightly higher than those determined in depth, respectively, at a mid-lake station (Ml, z = fixed samples. One maximum of very small heliozoans, 147 m). The latter station has been used as the routine however, could be detected only by the live counting sampling station by the Limnological Institute Con- procedure. These results will be reported in full detail stance since 1979 (e.g. Muller 1989, Tilzer et al. 1991, elsewhere (U. Zimmermann & T. Weisse unpubl.). Weisse 1991, Sommer et al. 1993). Live samples of 1 to The vertical and horizontal distribution of planktonic 3 1 volume were concentrated by gra.vity filtration heliozoans were investigated in fixed samples. Be- using a 10 pm plankton net. Aliquots corresponding to cause proper discrimination of genera was impossible Uberllngen l H4 I B Untere Giill Fig. 1. Study area with sampling locations. l Konstanz (A) Uberlinger See with routine mid-lake station (Ml) and sampling stations of the case study on the horizontal distribution 0 05 (H1 to Ha).(B) Obere Giill with routine lit- toral station (M21 and location of in situ growth rate measurements (W) Zlmmermann et al. Planktonic hel~ozoa 23 in fixed heliozoans we measured in Table 1 Hel~ozoangenera found In Lake Constance \wth thelr mean cell slze, those samples only total heliozoan mean cell volume and respective standard deviations. n number of indlv~duals measured number (vertical profiles) 01- abun- dance in 3 different size classes (<20pm, 20-30 \pm, and >30 pm, hol-1- Order Genus ('cl1 size Cell volume zontal distribution),respectively. Vei-ti- (1-lm1 (pm'1 cal profiles were recorded at the Actlnophry~da Actinophl-)/S (n - 185) 41.9 r S.5 46979 + 28179 pelagic station on August 17 and Octo- Centrohelida Raphjdocystls (n = 550) 25 0 t 5.0 9515 * 6153 ber 12, 1993. Samples were taken from Centrohelida Raphid~ophrys (n = 24) 18 8 r 2.9 3703 r 1591 discrete depths (0, 2, 4, 6, 8, 10, 15, 20, Centrohellda Heterophrys (n = 2605) 15 6 r 1.0 2065 r 524 25, 30, and 50 m),and total numbers of Centrohellda Chlamydaster (n = 195) 14 2 * 2.1 1668 r 793 Centrohellda Choanocystls (n = 526) 14.0 r 2.1 1599 r 730 heliozoans were counted by inverted Centrohellda Pterocystis (n = 79) 12 3 * 1.1 1053 r 277 mlcroscopy. The horizontal distribution of heliozoans was examined in fixed samples taken from the upper 8 m of the water column at 8 stations across Uberlinger See in ria were most important for taxonon~icidentification. early August (H1 to H8; Fig. 1).The coefficient of vari- Actinophrys: large cell size, central nucleus, absence ability (CV) of total cell numbers in fixed samples, de- of skeletal elements, large peripheral contractile vac- termined as given above for live samples, was 10 %. uole. Raphidocystis: 2 different types of spicules (tubu- In situ growth experiments were performed at Stn W lar and cup-shaped), and scales. Raphidiophrys: near the littoral station in summer and autumn. Water curved, spindle-shaped scales loosely attached to cell samples from 3 m depth were prefiltered through a body and basal part of axopods. Heterophrys: thick 140 pm mesh screen to exclude larger predators and gelatinous coat in which needle shaped spicules were then poured into diffusion chambers of 3.5 1 volun~e embedded.