Reciprocal Caulerpa Invasion: Mediterranean native Caulerpa ollivieri in the Bahamas supported by human nitrogen enrichment

The genus Caulerpa is known for its invasion of tropical, subtropical, and temperate coastal waters. Whereas the role of humans as vectors for the introduction of Caulerpa has been well documented, other anthropogenic factors that may mediate the success of an invasion are poorly understood. We provide evidence that a recent invasion of Caulerpa ollivieri into shallow Bahamian seagrass meadows is facilitated by anthropogenic nitrogen enrichment from sewage. Considering the accelerating nitrogen enrichment of coastal waters worldwide, our results suggest that reduction of anthropogenic nitrogen inputs must be achieved as a means of controlling similar biotic invasions. Humans are recognized as the primary vector in the global epidemic of biotic invasions in aquatic ecosystems (Carlton and Geller 1993). Much less is known about how anthropogenic modification of ecosystems facilitate biological invasions. We provide an example of how anthropogenic nitrogen enrichment has supported a successful invasion of the green alga Caulerpa ollivieri in coastal waters of Green Turtle Cay, Abacos, Bahamas. The islands of the Bahamas are surrounded by carbonate-rich subtropical waters that were historically oligotrophic and contained healthy coral reefs and nutrient-limited tropical seagrass ecosystems (Short et al. 1990). The growing resident and tourist populations on Green Turtle Cay, like many small island states globally, are increasing land-based nutrient loads to coastal waters from a variety of sources, especially untreated (raw) and/or partially-treated (septic tanks) domestic sewage. Recent studies have shown that the inshore waters of Green Turtle Cay are experiencing nutrient enrichment and eutrophication primarily as a result of localized sewage pollution (Barile 2001). Common symptoms of nutrient pollution in shallow, tropical and subtropical seagrass meadows include macroalgal blooms and epiphytization of seagrass blades, which reduce light and cause fragmentation and die-off of seagrasses, especially turtle grass, Thalassia testudinum (NRC 2000). Historically, T. testudinum meadows surrounding Green Figure 1. A) A productive turtle grass (Thalassia testudinum) meadow in nearshore waters of Green Turtle Cay, Abacos, Bahamas. B) Bloom of a matforming filamentous green macroalga (Cladophora sp.) causing fragmentation and die-off of T. testudinum in the Town Harbor, Green Turtle Cay. C) Microfilamentous algal epiphytes on blades of T. testudinum in Black Sound, Green Turtle Cay. D) Invasive bloom of Caulerpa ollivieri (denoted by arrow) causing fragmentation of and die-off of a T. testudinum meadow in Black Sound, Green Turtle Cay. continued on p. 3