J. Great Lakes Res. 8(1):169-183 Internat. Assoc. Great Lakes Res., 1982 SEASONAL ABUNDANCE PATTERNS OF DIATOMS ON CLADOPHORA IN LAKE HURON R. Jan Stevensonl and E. F. Stoermer Great Lakes Research Division University of Michigan Ann Arbor, Michigan 48109 ABSTRACT. Rocks bearing Cladophora were collected from May to November 1979 at two locations near Harbor Beach, Michigan, in Lake Huron to document seasonal patterns ofepiphytic diatom abundance and diatom proportion ofthe Cladophora-epiphyte assemblage biomass in an area receiving effluent from a municipal wastewater treatment plant. Data were examinedfor evidence of interactions between epiphytic diatoms and Cladophora. Cladophora first appeared in May at which time epiphytic diatoms comprised about 30% ofthe Cladophora-epiphytic assemblage biomass. Cladophora growth was greatest in June and July, accumulating much faster than diatoms. Peak Cladophora-epiphyte assemblage biomass was maintained from July to September. Cladophora biomass apparently decreased after August while diatom abundance increased to a September maximum. Diatoms were responsible for the sustained peak ofCladophora-epiphyte assemblage biomass as diatoms comprised over 60% ofthe assemblage biomass in September. The Cladophora-epiphyte assemblage collapsed by October. Low diversity of the epiphytic diatom taxocene and low diatom proportion of the Cladophora­ epiphyte assemblage biomass indicated Cladophora may have outcompeted diatom epiphytes during the June-July Cladophora growth pulse. Subsequently epiphytic diatoms may have enhanced Cladophora sloughing by shading and successful nutrient competition. INTRODUCTION problems associated with Cladophora, some epi­ phytes of Cladophora are important components of Epiphytes associated with Cladophora contribute energy flow in aquatic biological systems. Epiphytic substantial biomass to shoreline nuisance algal diatoms and bacteria are among the most efficiently growths. Bacteria, algae, protozoa, and inverte­ assimilated foods for some invertebrates (Hargraves brates accumulate upon the long filaments of 1970). Gammarus pulex L. and Asselus aquaticus Cladophora. Taft and Kishler (1973) estimated that L. graze diatoms from Cladophora filaments in Cladophora increased the surface area available for several rivers (Moore 1975). Stalked protozoa algal colonization by 4,000 km2 in an 18 km2 area commonly occur in substantial numbers (Taft and of the Ohio Islands region of western Lake Erie. Kishler 1973) and probably feed upon the detrital­ Epiphytic organisms colonize Cladophora as bacteria-algal epiphytes. Snails fed diatoms had heavily as any other filamentous alga. The absence greater fecundity than those fed Cladophora (R. of mucilage on filaments of Cladophora facilitates Patrick, Acad. Nat. Sci. Phil., pers. comm.). epiphyte attachment. The thick growths and highly Epiphytes were major components of aquatic insect branched thallus of Cladophora provide a refuge diets in many studies cited by Soszka (1975). The that reduces removal of epiphytes that could result minnow Chrosomus erythrogaster also effectively from wave disturbance. The standing crop biomass grazed epiphytes in a Minnesota stream (Phillips of diatoms on Cladophora can be greater than 1969). biomass of Cladophora itself in brackish water A substantial amount of research has considered (Jansson 1967) and in the Great Lakes (Stoermer, macrophyte-epiphyte interactions. Mucilage cover unpublished data). of the host substrate surface may decrease as the In addition to enhancing the potential nuisance plants become older, thereby enabling firmer at­ tachment of epiphytic organisms (Ballantine 1979). I Present address: Department of Biology, University of Louisville, Louisville, KY 40292. Precipitation of CaC03 by Potamogeton inhibited 169 170 STEVENSON and STOERMER epiphytism (Cattaneo 1978). Fitzgerald (1969) re­ ONTARIO ported low epiphyte crops on Cladophora in low nitrogen waters because Cladophora acted as a nitrogen sink. Nutrient and dissolved organic carbon exchange between the host substrate and epiphytes has been documented. Allen (1971) demonstrated macro­ phyte release of dissolved organic carbon in sub­ stantial amounts in a form usable by epiphytic algae and bacteria. Harlin (1973) documented epiphyte uptake of carbon and phosphorus released MICHIGAN from the macrophyte host. McRoy and Goering (1974) documented nitrogen transfer from host to epiphytes and explained the presence of large ONTARIO epiphyte standing crops in nitrogen-poor waters by nitrogen transfer after uptake from sediments by the host. Penha1e and Thayer (1980) recorded epiphyte uptake of 15 to 100% of phosphorus released by the host. Cattaneo and Kalff (1979) found greater alkaline phosphatase activity by FIG. 1. Sampling locations in Lake Huron near Harbor epiphytes on artificial than natural plants. They Beach, Michigan. suggested macrophytic supplementation of the phosphorus supply available to epiphytes, even Beach municipal wastewater treatment plant is though epiphyte biomass did not increase. discharged to Lake Huron via Spring Creek. Epiphytes also have an impact upon their host Samples were collected at sites 0.14 km south substrate. Harlin (1973) demonstrated transfer of (Location 4) and 0.18 km north (Location 6) of the phosphorus and carbon from epiphytes to host. stream mouth. Additional, special-purpose samples Howard-Williams, Davies, and Cross (1978) were made at a site 0.09 km south of Spring Creek showed that bacteria pitted the surface of (Location 4.5). Potamogeton leaves. Host photosynthesis was re­ Rocks bearing Cladophora were collected from tarded by reduced diffusion of HC03 to heavily Locations 4 and 6 on 14 May, 2 June, 28 June, 12 epiphytized leaves in low ambient HC03 environ­ July, 25 August, 18 September, 19 October, and 15 ments (Sand-Jensen 1977). Shading of the host November 1979. The rocks were in water between plant by epiphytes also reduced photosynthesis 0.25 and 0.75 m deep. Rocks were dipped once in (Sand-Jensen 1977) and reduced growth of macro­ lake water to remove the loosely associated silt­ phytes in more nutrient-enriched waters (Eminson diatom matrix, placed in bags, and stored frozen. and Phillips 1978). After thawing, the area covered by Cladophora was The objectives of this study were to evaluate outlined with a grease pencil for subsequent plani­ population and taxocene patterns of diatoms on meter measurement. Cladophora and epiphytes Cladophora as a function of season (May to were picked from the rock surface and placed in a November) in an area impacted by wastewater crucible for biomass determination. treatment plant effluent. Another purpose was The area outlined on the rocks was waxed. After estimations of the diatom epiphyte proportion of cooling, the wax was removed from the rock with the Cladophora-epiphyte assemblage biomass with attention focused on not stretching the wax form. respect to the temporal variable. Evidence which The wax form was traced on a piece of paper. Area suggests interactions between diatom epiphytes and of rock surface covered by Cladophora was deter­ Cladophora is presented and discussed. mined by planimeter measurement of the traced area (Fox et al. 1969). Dry weight and ash-free dry weight were deter­ METHODS mined for each Cladophora-epiphyte sample. The The study area was located at Harbor Beach, weight of each crucible was measured and sub­ Michigan on Lake Huron near the mouth of Spring tracted from the weight of crucible and Cladophora Creek (Figure 1). Treated effluent from the Harbor assemblage after drying in an oven at lO5°C for 24 DIATOMS ON CLADOPHORA IN LAKE HURON 171 hours. After ashing the sample at 500°C in a muffle epiphyte biomass, and diatom taxocene diversity. furnace, rewetting to replenish water of hydration, Epiphyte abundance was calculated as In cells/ cm2 and again drying the sample at 105° C for 24 hours, of rock surface area. Diatom profusion was cells/ g the ash-free dry weight of samples was calculated as total assemblage ash-free dry weight. Diatom the difference between the ash weight plus crucible proportion of assemblage biomass of a sample was weight and the dry weight plus crucible weight. estimated as the June-Location 4.5 average diatom Samples were then prepared for enumeration of ash-free dry weight times the number of diatoms in the epiphytic diatom association. The ash of a sample divided by the ash-free dry weight of the samples was placed in 150-mL tall beakers with 100 Cladophora-epiphyte assemblage (or average mL of nitric acid and boiled for 3 hours to loosen diatom biomass times diatom profusion). Species aggregates. The silt-diatom frustule suspension was diversity (Shannon and Weaver 1949) and evenness repeatedly settled and the supernatant siphoned (Hurlbert 1971) were computed for each diatom from the beaker to remove nitric acid. Aliquots of taxocene. the acid-free homogenized suspension were placed Throughout this paper, the term assemblage on coverslips, dried, and mounted on slides with refers to Cladophora and all epiphytic organisms. HYRAX®. Taxocene is defined as the group of all diatom Two slides were prepared from each sample and populations. Population refers to all individuals of valves of diatom taxa along two transects were a single diatom species or variety. Use of these term enumerated from each slide to check sample follows guidelines set by Hutchinson (1967). preparation variation. At least 500 valves were FIDO programs (data base management system identified