Nonindigenous plants are stressors of aquatic ecosystems.  Many species can form large monocultures that alter the abundance and diversity of the native flora or disturb physical aspects such as water flow, light penetration and dissolved oxygen concentration.  While not all introduced aquatic plants convey extreme impacts, the consequences of most introductions have yet to be understood.  Fundamentally, the establishment of nonindigenous plants preempts habitat for native species. As new taxa are introduced and the range of previously introduced species increases, specific knowledge of their distribution and potential range is imperative for resource management.

Thirty nonindigenous aquatic plant species representing twenty-four families are catalogued herein as established or present in the thirteen states comprising Region 5.  Sixteen species grow as emergent plants, six grow floating, and eight grow submersed (Figure 1). All thirty species originate from every continent except Antarctica (Figure 2). Species native to more than one continent, therefore, are represented as such. Two tropical species Pistia stratiotes (water lettuce) and Eichhornia crassipes (water hyacinth) appear to pose little threat to Region 5 ecosystems. However, others such as Myriophyllum spicatum (Eurasian water milfoil) and Potamogeton crispus (curly pondweed) are more climatically adapted and aggressively persistent. Two species, Cabomba caroliniana (Carolina fanwort) and Myriophyllum heterophyllum (two-leaf water milfoil), are native to North America and have been introduced to the Mid-Atlantic region. New York hosts the highest number of introduced aquatic plant species (21); Maryland and New Jersey each have 19 (Figure 3).

Figure 1. Growth form categories of aquatic plant species introduced to Region 5.

Figure 2. Origin of aquatic plant species introduced to Region 5.

Figure 3. Total number of nonindigenous aquatic macrophytes introduced by state within Region 5.

Long time introductions Iris pseudacorus (yellow iris), Lythrum salicaria (purple loosestrife) and Nasturtium officinale (watercress) are widely distributed in all thirteen Region 5 states.  Myriophyllum spicatum and Potamogeton crispus occur in twelve (Table 1). In the last thirty years, more new species have appeared in Region 5 than in the earlier 30 year time frames designated since 1850 (Figure 4). Hydrilla verticillata (hydrilla), Egeria densa (Brazilian waterweed) and a newly described hybrid form of Myriophyllum heterophyllum are highly invasive and currently expanding to new territories within the region.  The most recent introductions; Glyceria maxima (reed mannagrass), Glossostigma diandrum (mudmat) and Marsilea mutica (nardoo) are still limited in range.

Figure 4. Introductions of aquatic plant species to Region 5 states over forty-year time increments from 1850 to present. Bars represent the number of species making their first appearance in a given state. Quantities are not cumulative but indicate the number of new introductions per time increment.  Species are added either as range expansions from adjoining states or new introductions from outside of the region. The earliest introductions, Lythrum salicaria and Nasturtium officinale were recorded in 1831.

Species are categorized as submersed, emergent, or floating plants.  Each is detailed in an account that includes a chart indicating the earliest known occurrence in each state.  Dates with an asterisk (*) indicate that the actual year of introduction is not known. The date provided is the publication date of the citation where this species was first recorded.  Additional records of nonindigenous species from refuge or other lands and waters are welcomed at: http://nas.er.usgs.gov/reportcol.htm.

Table 1. Nonindigenous aquatic plant species recorded in states of Region 5.