Rex L. Lowe

Algal periphyton growth on nutrient-diffusing substrates: an in situ bioassay

Differences in nutrient limitation for dominant species within an algal periphyton com? munity were determined using additions of N and P supplied by nutrient-diffusing artificial substrates. Sealed clay flowerpots were filled with 2% agar and one of nine nutrient treatments (all combinations of K2HP04 at 0.0, 0.05, and 0.5 mol/L with NaN03 at 0.0, 0.05, and 0.5 mol/L). The pots were submerged at 0.5 m depth in Douglas Lake, Michigan, and diffused N and P to their outer surfaces in proportion to internal concentrations. After 51 d the pots were scraped and analyzed for attached algae. Total algal biomass as chlorophyll a on the pots ranged from 0.17 ? 0.02 (se) ^g/cm2 for pots without added nutrients to 15.7 + 2.0 /xg/cm2 for pots with K2 HP04 at 0.05 mol/L and NaN03 at 0.5 mol/L. Chlorophyll a on pots containing just P (0.05, 0.5 mol/L) increased 6- to 10-fold over controls. The diatoms Epithemia adnata and Rhopalodia gibba and the blue-green alga Anabaena in? creased significantly on the P-only pots; these species are suspected of N-fixing capability. Chlorophyll a on pots containing just N (0.05, 0.5 mol/L) increased 1.5- to 2-fold, though this increase was nonsignificant; Achnanthes minutissima, Gomphonema tenellum, and Cocconeis placentula showed enhanced growth on these pots. Combinations of N and P caused heavy growths of the filamentous alga Stigeoclonium tenue. Naviculoid diatoms were also most abundant on the N + P pots. Average nutrient levels in Douglas Lake during the study were: NH3, 2.02 /nmol/L; N03, 0.44 Aimol/L; and P04, 0.06 /miol/L. The low ambient concentrations of both N and P, together with results ofthe periphyton bioassay, indicate that the two nutrients may jointly limit overall growth, and that the form of growth limitation differs by species within the periphyton community.

Periphyton Response to Nutrient Manipulation in Streams Draining Clearcut and Forested Watersheds

Nutrient-releasing artificial substrata were deployed in streams draining clearcut and forested watersheds to evaluate resources potentially limiting to populations of benthic algae. Nitrogen, phosphorus, and calcium were released singly and in combination in the two streams that differed primarily in light availability. Periphyton were harvested after one- and two-month exposure periods and analyzed for chlorophyll. The two-month substrata were additionally analyzed for algal community structure. Algal periphyton in the clearcut stream accumulated more chlorophyll and biovolume than in the forested stream across all nutrient treatments. Algal community structure was significantly different between streams but not between nutrient treatments. Algal physiognomies were also significantly different between streams with filamentous green algae dominating the clearcut stream and erect diatoms dominating the forested stream. Light appears to limit algal accumulation in the forested stream and there is evidence that some populations in the clearcut stream may be nutrient limited. Adequate light also resulted in a more architecturally diverse community.

Quantifying mechanisms for zebra mussel effects on benthic macroinvertebrates: organic matter production and shell-generated habitat.

Quantitative descriptions of interactions between zebra mussels (Dreissena polymorpha) and other organisms are needed for an understanding of zebra mussel effects on energy flow and community dynamics in North American freshwater ecosystems. We used a field experiment to quantify effects of 2 potential mechanisms for increasing benthic macroinvertebrate biomass and densities in the Great Lakes following the zebra mussel invasion: 1) habitat created by zebra mussel shells, and 2) habitat and food provided by increases in benthic organic matter. Macroinvertebrate biomass, macroinvertebrate densities, and organic matter mass (hereafter termed organic matter) were measured on substrates (tiles without zebra mussels, with zebra mussel shells, and with live zebra mussels) that were held in western Lake Erie for 37 d. Organic matter was 3×-5× greater on tiles with live mussels than on plain tiles or tiles with shells, demonstrating that filter-feeding zebra mussels increased organic matter on benthic substrates. Organic matter did not differ on tiles with shells and tiles without mussels. Similarly, total macroinvertebrate biomass was 2×-5× greater on tiles with live zebra mussels than on tiles with shells and tiles without mussels; however, macroinvertebrate biomass also was 3X greater on tiles with shells than on tiles without mussels. Habitat created by shells was the most important cause for zebra mussel-generated increases in total macroinvertebrate biomass and densities of Hydridae, Dugesia tigrina (Turbellaria), Physella sp. (Gastropoda), Gammarus fasciatus (Amphipoda), and Microtendipes pedellus (Chironomidae). However, organic matter produced by zebra mussels also caused increases in total macroinvertebrate biomass and densities of Physella and Microtendipes. Zebra mussels did not reduce densities of any macroinvertebrate taxon. Because benthic macroinvertebrates are important prey for crayfish and benthivorous fishes, increases in macroinvertebrate abundance may be important in channeling energy from zebra mussels to higher trophic levels in the Great Lakes.

Shifts in Benthic Algal Community Structure and Function Following the Appearance of Zebra Mussels (Dreissena polymorpha) in Saginaw Bay, Lake Huron

Zebra mussel, Dreissena polymorpha (Pallas), proliferation in Saginaw Bay, Lake Huron is associated with increased water clarity and increased light levels on benthic substrata in the littoral zone. We hypothesized that the filtering activities of Dreissena and associated increases in light penetration should affect the structure and function of benthic algae in the bay. Monthly quantitative benthic algal samples were collected from natural substrata by SCUBA in the littoral zone of the bay through the growing seasons of 1991 (initial Dreissena colonization), 1992 and 1993 (post-Dreissena colonization). Algal community structure was examined microscopically and productivity rates were measured using carbon-14 in sealed acrylic chambers in situ. Our data demonstrate that, following Dreissena proliferation, light penetration, benthic algal biomass, chlorophyll concentrations and rates of benthic primary productivity have increased. These changes coincided with a shift from diatom domination of the benthic algal community to a flora dominated by filamentous green algae (Zygnematales). We suggest that these shifts have the potential to affect benthic food webs within littoral zones of the Great Lakes.

Nutrient limitation of algal biomass accrual in streams: seasonal patterns and a comparison of methods

In-situ, nutrient amendment experiments (nutrient-diffusing substrata, NDS) were conducted in 12 New Zealand gravel-bed streams to investigate seasonality of biomass accrual and nutrient limitation of benthic algal communities. Benthic algal biomass accrual rates exhibited significant (p = 0.019, repeated measures ANOVA) seasonal differences; rates were greatest in summer and least in winter. The degree of nutrient limitation also differed (p = 0.003) seasonally; periphyton community biomass was most responsive to nutrient amendments in summer and least responsive in winter. Temperature may be the underlying cause of these patterns. The ratios of dissolved inorganic nitrogen to soluble reactive phosphorus (DIN:SRP) in streamwater and of streambed periphyton communities were of limited use for predicting which nutrient limited NDS bioassays; cellular nutrient content was weakly predictive. This study demonstrates the need to consider temporal changes (i.e., seasonality) when assessing the influence of nutrients on stream ecosystems, and indicates that the use of nutrient ratios to ascertain which nutrient may limit benthic algal biomass should be validated with field experiments.

Artificial substrates which release nutrients: Effects on periphyton and invertebrate succession

Nutrient-diffusing substrates for periphyton were made from clay flower pots (O.D. = 8.8 cm), sealed with plastic petri dishes, and filled with 2% agar and specified nutrients. When placed in water, the nutrients slowly diffuse through the agar and clay walls of the pots, becoming available to organisms colonizing the outer surface.Forty-eight pots, 16 containing 0.1 M KH2PO4, 16 with 0.1 M NaNO3, and 16 with no added nutrients, were placed at 0.5 m depth in Douglas Lake, Michigan. Four pots of each nutrient treatment were sampled for algal periphyton and invertebrates after 7, 14, 25, and 36 days. A total of 72 algal species were enumerated. Of these, Epithemia adnata (Kütz.) Bréb., Rhopalodia gibba (Ehr.) O. Müll, and Anabaena sp. experienced strong growth stimulation in response to phosphate addition. No significant effects of nitrate addition were noted. Measures of algal community structure also reflected the impact of phosphate addition. Final algal biomass was enhanced 10-fold, successional change was prolonged, and species diversity declined relative to nitrate and control pots. Chironomids, chydorid Cladocera, and gastropods dominated the invertebrate fauna found on the pots. Densities of the small, algivorous chironomid Corynoneura nr. lobata Edwards were highest on phosphate pots at the end of the study, in apparent response to the increase in periphyton biomass.

Macroinvertebrate Communities on Hard Substrates in Western Lake Erie: Structuring Effects of Dreissena

Of the approximately 140 non-indigenous species that have invaded the Great Lakes since the early 1800s, few have had greater effects on this ecosystem than the zebra and quagga mussel (Dreissena polymorpha and D. bugensis, respectively). In this study the effects of these bivalves on macroinver-tebrates inhabiting hard substrates in western Lake Erie were quantified. Biomass, densities, diversity of macroinvertebrates, and paniculate organic matter mass were measured on bricks with high and low Dreissena densities that were held at a depth of 3.5 mfor 49 days in 1996. Total macroinvertebrate densi ties and biomass (excluding Dreissena) were two and five times greater on substrates with high Dreissena densities than when Dreissena were rare. These differences were largely attributed to the amphipod Echinogammarus ischnus, itself an invading species, which constituted 29 and 31% of total macroinvertebrate densities and biomass, respectively, on Dreissena-dominated substrates. Dreissena also stimulated increased macroinvertebrate diversity, causing a shift from a community dominated by the chironomid Dicrotendipes neomodestus to an assemblage characterized by increased densities and equitability of hydroids (Hydridae), the flatworm Dugesia tigrina, tubificid oligochaetes, leeches (Alboglossiphonia het-eroclita and immature Erpobdellidae), limpets (Ancylidae), snails (Physella integra and Amnicola limosa), Echinogammarus, the microcaddisfly Hydroptila ?waubesiana, and the chironomid Micro-tendipes pedellus. No taxon responded negatively to Dreissena in this study. In addition, particulate organic matter mass, an important food and habitat resource for benthic invertebrates, was two times greater on Dreissena-dominated substrates than on bricks with few Dreissena. Our results support hypotheses that Dreissena have strong effects on community dynamics and energy flow pathways in the Great Lakes.

SPATIAL AND TEMPORAL DISTRIBUTION OF BENTHIC DIATOMS IN NORTHERN LAKE MICHIGAN

Benthic diatom communities were sampled from natural substrates along a depth tran- sect in northern Lake Michigan to examine spatial and seasonal distribution patterns and define assemblages of associated taxa. Information-theory diversity, evenness, and richness were signifi- cantly higher in deep assemblages than in shallow assemblages, with the boundary falling between 5 and 10 m in winter and fall, 10 and 20 m in spring, and 20 and 31 m in summer. Planktonic taxa are abundant on benthic substrates in the deep zone, primarily as dead fallout. In shallow water, large numbers of living planktonic taxa overwinter beneath the ice on benthic substrates. Cluster analysis of samples was used to identify and define diatom assemblages from deep, mid- depth, and shallow water. The deep-water assemblage contains motile and nonmotile benthic species that have been reported from alpine and boreal oligotrophic lakes. The mid-depth assemblage is rich in epipsammic taxa. The shallow-water assemblages contain cosmopolitan taxa that are collected often in nearshore plankton samples. These shallow assemblages do not persist throughout the year as do the deep and mid-depth assemblages. Cluster analysis of taxa provides further definition of species associations and the degree of cor- relation among species. Correlations among benthic taxa are strengthened when planktonic taxa are removed from the analysis, indicating that plankton fallout represents an addition of information which is poorly related to distribution of the benthic species. The most stable and diverse community is found at the deepest sampling depths where environ- mental conditions are most stable, the mid-depth community exists in fluctuating but predictable conditions, and the shallow assemblages occur in the zone of maximum physical fluctuation or natural stress. Substrate types are correlated with the species assemblages, but the high diversity of substrate types in shallow water does not support the most diverse assemblage. The discrete species assem- blages seen in an oligotrophic area of this large dimictic lake apparently respond sensitively to tem- porally and spatially dynamic environmental parameters.

Benthic Stream Algae: Distribution and Structure

Abstract Algae are ubiquitous in streams and play fundamentally important roles as primary producers at the interface of the physical–chemical environment and the biological community. In this chapter, we present an introduction to the taxonomic and ecological classification of stream algae for the investigator with little previous experience with algae. You will learn to recognize stream benthic algal growth in a variety of microhabitats and how to identify common North American stream algae using an illustrated key. You will also be introduced to the ways by which different ecological factors structure the stream algal community by investigating differences in algal density and biovolumes among different microhabitats.

COMPARATIVE ULTRASTRUCTURE OF THE VALVES OF SOME CYCLOTELLA SPECIES (BACILLARIOPHYCEAE)1

Cyclotella atomus Hust., C. meneghiniana Kütz., C. comta (Ehr.) Kütz., C. antiqua W. Sm., C. michiganiana Skv., C. ocellata Pant., C. glomerata Bachmann, C. pseudostelligera Hust., and C. stelligera (Cleve et Grun.) V. H., were selected for ultrastructure investigations on the basis of their ready availability. Specimens were examined employing both transmission and scanning electron microscopy. Strutted processes, labiate processes, spines and other special features were surveyed in all species. On the basis of these observations the species of Cyclotella observed seem to be in one of 3 morphological groups, the meneghiniana group, the comta group, and the stelligera group.