| Population increases of macroalgae have been documented on coral reefs in the Caribbean over the past 2 decades, leading to a hypothesized decrease of ecosystem health. To gain a better understanding of reef system dynamics, this study focused on the natural history, growth, and reproduction for shallow (7m) and deep (21–27m) Halimeda tuna (Chlorophyta) and Dictyota spp. (Phaeophyta) populations in a relatively undisturbed reef setting over a period of 7 years. For both genera, overall population numbers fluctuated on a yearly basis, but no steady increase or decrease of population numbers was observed. For H. tuna, shallow, back reef areas support significantly higher population densities, but exhibit lower productivity than deeper reef slope populations. Despite this phenomenon, the high number of plants found at shallow locales made these areas more important in terms of CaCO3 and sediment production. Unlike Halimeda, native weedy species such as Dictyota can show population-based responses similar to those of invasive species when space is opened after physical disturbance. In the marine environment, turbulent water motion generated by hurricanes reduces densities and physically injures many marine organisms. In the Florida reef tract, Hurricane Irene shredded populations of the weedy algae Dictyota spp. at Conch Reef into small fragments as intact adult thalli were torn from the substratum. Immediately following hurricane impact, numbers of Dictyota spp. fragments in the water column over sites at two depths was four-fold greater than one month after the storm. Despite the trauma of fragment formation, hurricane-generated fragments of Dictyota spp. as small as 3 mm remained viable, and attached to sand within 48 hours. One month after Dictyota spp. populations were drastically reduced by hurricane forces, percent cover rapidly recovered to half of pre-hurricane population densities, even as light levels and daylengths declined during winter months. In summary, this study shows that many natural forces regulate populations of macroalgae in the Florida Keys, and population parameters such as growth, sexual and asexual reproduction, and longevity on relatively unimpacted reefs need to be understood in order to understand when and if anthropogenic activities are the source of reef ecosystem changes. |
