121 Threats to the biodiversity of northeastem North American lakes: aquariums and garden ponds Paul H. Lord 1 ABSTRACT Many nuisance aquatic plants (e.g., Eurasian water-milfoil, water chestnut, and Brazilian elodea) have been introduced to Northeastern North American (NE) waters by aquarium enthusiasts. The growing enthusiasm for water gardens provides significant potential for new exotics escaping into our lakes. Federal and New York State (NYS) laws require proof of damage to the environment prior to initiating action to regulate potential introductions. Since garden ponds make more common use of temperate exotics than do aquariums, new exotics will be introduced into NE lakes unless preemptive action is taken. BACKGROUND Twentieth century changes in NE lakes have been largely associated with the introduction of non-native species. Lampreys, alewives (Warner, 1999), zebra mussels, Eurasian water-milfoil, and purple loosestrife have all had profound impacts on NE lacustrine environments (Sinnot and Paul, 1993; Harman et al., 1997; Claudi and Leach, 2000). Macrophytes, large aquatic plants attached to the bottom, are important lacustrine community members. They are competitors with planktonic algae for dissolved nutrients. By removing nutrients (particularly phosphorous which is most often limiting in NE lakes) from the water, macrophytes limit algae growth and improve water clarity (Harman et aI., 2000). Macrophytes are restricted to relatively shallow water by pressure and light penetration (Hutchinson, 1975), but their growth in these shallow waters helps to trap nutrients as the nutrients enter a lake. Macrophyte shallow-water growth also helps to stabilize lake bottoms minimizing the mixing ofbottom sediments into the water column that can occur when boats or winds stir shallow waters. Additionally, these plants help to protect lake shorelines from erosion. Most importantly, macrophytes provide food, cover, and oxygen for a diverse littoral community. Their composition often regulates the numbers and kinds offish and waterfowl found in a body ofwater (Harman et al., 2000). Healthy macrophyte communities are comprised of a variety of species. When viewed underwater, their canopied nature is normally apparent. Taller more filigreed plants tower over shorter, stockier plants. Each plant has a strategy for catching a portion 1 SUNY Oneonta biology graduate student. Contribution for Biol. 586, Conservation of Biodiversity. Present address: 100 Sunset Ridge, Cooperstown, N.Y. 13326; E-mail: [email protected]; telephone: (607) 547-5962. 122 of the sun's energy. In some healthy lakes the bottom is more patchy than canopied, i.e., there are locations without a significant mix ofplants, but the lake overall has a mixture ofplants extending throughout its littoral zone (Personal observation). Increasingly, NE lakes are dominated by aggressive exotic species NEAPMS, 2000; FOLA, 2000). Exotic species are those species not native to an ecosystem. Aggressive exotics are those that have not only survived introduction into the ecosystem, but have attained a dominant position in the community changing that ecosystem significantly. A common situation in lakes in the northeast United States is that Eurasian Water­ milfoil (Myriophyllum spicatum) [EWM] dominates a lake after introduction. Formerly mixed communities involving Potamogeton spp., Elodea, Vallisineria, Ceratophyllum and one or more ofthe macroalgae (Chara or Nitella) are replaced by nearly monoclonal stands of the mostly submerged EWM. EWM grows from hearty roots firmly established in the substrate. Over the winter months it stores significant energy in those roots and in a few shoots that persist under ice. The persistent shoots tower over most of the competition in the spring and facilitate its early physical dominance over most of the native species. EWM continues to grow towards the surface in early summer. Once it has reached the water's surface it grows along the surface in dense mats shading out all competitors beneath it (Anonymous, 1993; Johnson, undated; personal observation). EWM can reproduce sexually, but is most often spread to new areas by fragments (asexual reproduction) via animals, people, boats, and trailers (Hellquist, 2000b; Johnson, 2000). One submerged plant that achieves early seasonal dominance over EWM is curly leaved pondweed (Potamogeton crispus). This is an exotic species from a genus widely distributed in the U.S. This plant can be a serious pest in southern New York as well as over much of the rest of the NE (Skogerboe, 2000; personal observation). It reproduces asexually with turions. Many ofthese turions bud as other plants die back in the autumn (Hellquist, 2000b; personal observation). The rest bud soon after ice out (personal observation). As a lake warms in spring and early summer these plants grow straight to the surface where they form dense mats which grow across the surface. In most areas of the NE, curly leaved pondweed dies back in the first or second week of July (when it is often replaced by EWM), but in some New York lakes bordering New Jersey, it persists in maintaining dense mats throughout the summer months (Anonymous, undated(a); Anonymous, 1997b; Hellquist, 2000b; Schmidt and Kannenburg, 1998; personal observation). Perhaps the most detested exotic macrophyte found in NE is the water chestnut (Trapa natans). Water chestnut overwinters as a sizeable homed and barbed (edible) nutlet which lays on a lake bottom until spring. These nutlets are not the traditional water chestnut of Chinese cooking (Elocharis dulcis [Hellquist, 2000bD. They can persist for up to four years (Kishbaugh, 2000), possibly longer (Hellquist, 2000b). The nutlet produces a shoot which bears dissected underwater leaves which are not remarkably different from a number of native submerged macrophytes; however, as the plant 124 The threat is not only from outside our national borders. A number of species have been introduced from outside our region into our NE lakes. Fanwort, Cabomba caroliniana, moved up from Southeastern U.S. to the NE in 1932. Both common naiad, Najas guadalupensis, and variable water-milfoil, Myriophyllum hetrophyllum, are now in the NE and far from their Southwestern U.S. origins (Hellquist, 2000a, 2000b). At least one exotic believed to be tropical appears to be adapting to its U.S. environment by moving along the East Coast (Hellquist, 2000b; Anonymous, 1997b). Hydrilla, Hydrilla verticillata, is well established in Florida where it is the target of expensive control measures (Bowes, 1982; Wilet, 1984; Barret, 1989; Anonymous 2000i). It has moved into portions of the NE even while it continues to be sold as an aquarium plant. Its capability to adapt to our winters should not be underestimated since it is found in Siberia (Hellquist, 2000b). Perhaps the most recent threat to our NE waters is by mudmat, Glossostigma diandrum, which has been found in both Pennsylvania and New Jersey waters. This delicate looking flowering plant is native to Australia, New Zealand, India, and East Africa (Anonymous, 1998b). Not surprisingly, the problem is not one limited just to the NE, or to the U.S. (Rosenfeld and Mann, 1992). Almost every country developed enough to note ecological changes in their waters does note the impact of introduced aggressive macrophytes. As an example, Australia is dealing with a number of imported macrophytes and the problems associated with their introductions. Senegal tea plant, Gymnocoronis spilanthoides, was initially introduced into Australia from India as an aquarium plant in the 1970s. In several instances it has escaped and quickly taken over the water body into which it was introduced (Carter, 1994a). Alligator weed, Alternanthera philoxeroides, a native ofArgentina, was mistakenly cultivated by the Sri Lankan community in Australia because of its similarity to their native plant used as a leafy vegetable. It forms large persistent infestations in rivers (Gunasekera and Rajapakse, 1998). Hydrocotyle, Hydrocotyle ranunculoides, is a native to South America that has caused problems in the U.S., Israel, and Australia. It has shading floating leaves which grow rapidly and therefore obstruct the flow of water (Carter, 1994b). Additionally, the Australians are dealing with a U.S. import: American grass, Echinolchloa microstachya, is invading Australian rice fields (Randall, 2000). AGGRESSIVE EXOTIC MACROPHYTE CHARACTER Generalizations can be made about the character of aggressive exotic macrophytes (AEMs). AEMs reproduce vigorously, both asexually and sexually. The asexual reproduction makes removal difficult and facilitates plant spread. Most AEM plants can reproduce by fragmentation, i.e., they will grow complete new plants from a bit of stem or leaf in the course of a growing season. Such fragments easily separate from the original plant and are readily disseminated by man (principally on boats and boat trailers) and by animals such as waterfowl flying from one waterbody to another (Sinnot and Paul, 1993; Hellquist, 2000b; personal observation). Sexual reproduction assists survival over the course of harsh NE winters. By turions, seeds, nutlets, or other 125 propagules, the aggressive exotic macrophytes overwinter with substantial food reserves ready to provide a head start on spring growth (Hellquist, 1993; personal observation). AEMs have a variety of other characteristics that provide them an edge in NE lakes. They tend to be early light gatherers, i.e., they grow from their winter forms earlier than do the native