Benthic Algae of Lake Erie (1865-2006): A Review of Assemblage Composition, Ecology, and Causes and Consequences of Changing Abundance

Peer-reviewed literature and published reports were used to summarize knowledge of benthic algal assemblage composition and ecology in Lake Erie, and causes and consequences of temporal variation in algal abundance. Macroalgal assemblages in rocky littoral and so— substrate habitats have been reasonably well described and studied, as has the epiphyte/ metaphyte assemblage associated with rocky littoral macroalgae. In contrast, little information exists for non-epiphytic microalgae in littoral habitat. During the period when algal records were reported (1865-2006), the rocky littoral macroalgal assemblage was o— en dominated by the chlorophyte genera Cladophora and Ulothrix and the rhodophyte Bangia, whereas the charophytes Chara and Nitella were most abundant in littoral so— substrate. In addition to substrate eš ects, assemblage composition varied as a function of depth, temperature, light levels, and nutrient concentrations. Under certain conditions, macroalgal taxa appeared to outcompete and exclude other taxa from littoral habitat. However, these organisms have also facilitated increased algal diversity by supporting epiphytes/metaphytes. In Lake Erie, signi› cant temporal change in benthic algal abundance has been associated with: 1) eutrophication (prior to 1972), 2) oligotrophication following the Great Lakes Water Quality Agreement (1972-1985), and 3) invasion by Dreissena mussels (1986-2006). Increasingly eutrophic conditions were re¢ ected by high abundance and frequent shoreline fouling by Cladophora, declining charophyte (such as Chara, Nitella) abundance, and invasion of rocky littoral habitat by euryhaline Bangia. Subsequent indicators of oligotrophication included declining Cladophora abundance, and increased diatom abundance in deepwater habitat. Eš ects of › lter-feeding Dreissena (such as increased water clarity, phosphorus excretion) were likely causes for Cladophora resurgence in the 1990s, and likely contributed to return of Chara and Nitella to formerly occupied habitat. Algal assemblages clearly re¢ ect environmental conditions in aquatic ecosystems. To accurately assess present and future conditions in Lake Erie, continued study of all benthic assemblages is recommended, with greater attention directed toward microalgae in littoral habitat than has occurred in the past. OHIO J SCI 108 (5): 82-94, 2008 1Address correspondence to Timothy W. Stewart, Department of Natural Resource Ecology and Management, Iowa State University, Ames, Iowa 50011. Email: [email protected] INTRODUCTION Algae are aquatic, photoautotrophic, nonvascular plantlike organisms possessing chlorophyll a and unicellular reproductive structures (Stevenson 1996, Sheath and Wehr 2003). A variety of prokaryotic and eukaryotic, and unicellular and colonial organisms are included in this group, with taxonomic divisions distinguished on the basis of photosynthetic pigments, energy-storage products, and cell coverings (Sheath and Wehr 2003). Benthic algae, de¬ ned as algae living on or associated with submerged substrates, are critical functional components of freshwater ecosystems (Lowe 1996, Stevenson 1996). As primary producers and invertebrate food, these organisms regulate secondary production and nutrientand energy-È ow pathways in benthic food webs (Lowe 1996, Stevenson 1996). Benthic macroalgae also provide habitat for many organisms, including invertebrates, ¬ shes, and other algae and microorganisms (Lowe 1996, Stevenson 1996). Additionally, taxonomic composition of algal assemblages is a reliable indicator of water and habitat quality (Lowe and Pan 1996, Stevenson and Smol 2003). Lake Erie is the shallowest, southernmost, and most productive of the North American Great Lakes, and its extensive littoral (i.e., shallow) habitat, relatively warm water, and abundant nutrients are favorable to benthic algae (Bolsenga and Herdendorf 1993, Lowe 1996). Previous studies demonstrated that benthic algae are key determinants of energy and nutrient È ow pathways and biotic community structure in Lake Erie, and that their abundance a3 ects and reÈ ects environmental conditions in the lake (Knapton and Petrie 1999, Carrick and others 2005, Higgins and others 2008). In fact, of all Lake Erie biota, the benthic alga Cladophora has arguably been the most visible and inÈ uential biological indicator of water and habitat quality changes occurring since Europeans colonized the watershed (Shear and Konasewich 1975, McGucken 2000, Higgins and others 2008). Knowledge of benthic algal assemblage composition, abundance, and ecology is requisite to understanding environmental conditions in Lake Erie. However, the extent of knowledge on Lake Erie’s benthic algae isn’t readily apparent because existing information is scattered across a large time period and quantity of literature. In this paper, published information from benthic algal investigations in Lake Erie is used to 1) report records of benthic algal taxa from Lake Erie, 2) summarize knowledge of benthic algal ecology and assemblage structure, and 3) describe causes and consequences of temporal change in benthic algal abundance. ̈ e information base produced from this review is intended to assist in designing future algal-based investigations in Lake Erie, and in relating results of these studies to spatial and temporal variation in environmental conditions. An additional objective was to identify understudied benthic algal assemblages, and therefore facilitate investigations that ¬ ll knowledge gaps and improve understanding of Lake Erie ecology. METHODS A comprehensive survey of primary, peer-reviewed literature was attempted by obtaining any journal article thought to contain information on Lake Erie benthic algae. Information from accessible books and published reports that never appeared in a peer-reviewed journal was also included. In total, 838 publications were obtained, and records and accompanying ecological information are presented from all 87 references that contained original records of benthic algal genera or species from Lake Erie (Table 1). Necessary criteria for records to be included in this review included evidence that OHIO JOURNAL OF SCIENCE 83 T. W. STEWART AND R. L. LOWE organisms were alive at time of sampling, and obtained from a benthic habitat of Lake Erie proper (i.e., open lake, bay or harbor, moist substrate above the water line). Due to the historically unstable nature of species-level taxonomy for algae, and lack of extensive ecological information for most species in Lake Erie, discussion of assemblage composition, ecology, and abundance is focused at the generic level of taxonomic resolution. ̈ is is the ¬ rst comprehensive review of published records and information on benthic algae of Lake Erie. Previous reviews of algae that inhabit Lake Erie benthic habitat focused on speci¬ c taxa and were conducted across multiple ecosystems (Wolfe and Sweeney 1982, Sheath 1987, Higgins and others 2008).  e Lake Erie Ecosystem Lake Erie lies between 41o 21 ́N and 42o 50 ́N latitude and 78o 50 ́W to 83o 30 ́W longitude, and covers 25,734 km2 with maximum breadth and length of 92 km and 388 km (Bolsenga and Herdendorf 1993, Haltuch and others 2000). Mean and maximum depths are 19 and 64 m, and the lake holds 484 km3 of water (Bolsenga and Herdendorf 1993). ̈ e long axis of Lake Erie parallels prevailing wind direction, causing high waves (up to 3.6 m) and È uctuating water levels in response to storms (Bolsenga and Herdendorf 1993). Depending on conditions, winter ice cover ranges from 0-100 percent of surface area (Bolsenga and Herdendorf 1993). ̈ e lake is divided into eastern, central, and western basins (i.e., eastern, central, and western Lake Erie; Bolsenga and Herdendorf 1993, Haltuch and others 2000; Fig. 1A). ̈ e western basin is relatively small (4,837 km2 surface area, 25 km3 volume), shallow (mean and maximum depth of 7.4 and 21 m, respectively; Fig. 1B), and the most biologically productive per unit area (Bolsenga and Herdendorf 1993). ̈ ermal strati¬ cation in the western basin is only temporary due to frequent wind-driven mixing, whereas stable summer strati¬ cation occurs in remaining basins (Bolsenga and Herdendorf 1993). ̈ e rather È at-bottomed central basin is intermediate in size, depth (15,061 km2 surface area, 305 km3 volume, mean and maximum depth of 18.5 and 39 m; Fig. 1B), and productivity, and experiences hypolimnetic oxygen declines during summer months (Bolsenga and Herdendorf 1993, Haltuch and others 2000, McGucken 2000). ̈ e large, deep eastern basin (5,836 km2 surface area, 154 km3 volume, mean and maximum depth of 18.9 and 64 m; Fig. 1B) has the lowest summer temperatures and productivity, and experiences little if any hypolimnetic oxygen depletion (Bolsenga and Herdendorf 1993, Haltuch and others 2000, Carrick 2004). Spatial variation in benthic algal assemblage structure is heavily inÈ uenced by temperature, light, water È ow regime (such as current and wave action), depth, and substrate type, especially particle size and hardness (Lowe 1996, Wehr and Sheath 2003a). In Lake Erie, rock (i.e., bedrock, boulder, cobble) is the dominant substrate type in nearshore, or littoral regions of the eastern basin (Bolsenga and Herdendorf 1993, Haltuch and others 2000; Fig. 1C). Rock is also abundant along the south shore of the central basin, and in the island region of the western basin (Bolsenga and Herdendorf 1993, Haltuch and others 2000; Fig. 1C). Remaining substrate generally consists of clay, mud, sand, or gravel; although abundant glacial till along north shores of eastern and central basins, and southwest shores of the central and western basins, contains particles ranging from large rocks to clay or mud (Bolsenga and Herdendorf 1993, Haltuch and others 2000). Overall, approximately 85 percent of the lake bed is composed of soÌ substrate (i.e., ¬ ne sediments, including clay, mud, sand; Haltuch and others 2000). RESULTS AND DISCUSSION Macroalgae of Rocky Littoral Habitat ̈ e most thoroughly studied benthic algal assemblage of Lake Erie consists of ¬ lamentous, colonial, and plantlike growth forms that are oÌ en visible to the unaided eye and are attached to hard substrates in shallow, nearshore locations (i.e., rocky littoral habitat). Bedrock, boulders, and cobble are primary substrates, although other large substrate particles (such as gravel, wood, concrete, metal) are also colonized. Most rocky littoral habitat in Lake Erie is located along exposed shorelines subject to heavy wave action and substantial water movement (Bolsenga and Herdendorf 1993, Haltuch and others 2000). ̈ ree ¬ lamentous genera have frequently been reported to dominate biomass and areal cover in this habitat: the rhodophyte Bangia and the chlorophytes Cladophora and Ulothrix (Garwood 1982, Chilton and others 1986, Higgins and others 2008; Table 1). Additional macroalgal genera recorded in multiple surveys or that have occasionally been very abundant include the cyanophytes Calothrix, Lyngbya, Oscillatoria, Phormidium, and Plectonema, the rhodophyte Chroodactylon, and the chlorophytes Hydrodictyon, Rhizoclonium, Stigeoclonium, and Tetraspora (Pieters 1902, TaÌ 1964, TaÌ and Kishler 1968, Downing 1970, TaÌ and Figure 1. Lake Erie maps showing (A) key landmarks and other geographic locations, (B) depth contours (in m), and (C) bottom areas covered by rock substrate (indicated by gray shading). Maps were constructed using information from Bolsenga and Herdendorf (1993), Haltuch et al. (2000), and McGucken (2000). 84 VOL. 108 BENTHIC ALGAE OF LAKE ERIE