Persistence and growth of the fecal indicator bacteria enterococci in detritus and natural estuarine plankton communities

Abstract

Enterococci are used to evaluate recreational-water quality and health risks in marine environments. In addition to their occurrence in feces of warm blooded animals, they are also common epiphytes. We investigated the contribution of plankton- or particle-associated enterococci in estuarine and coastal water. Seven water and size-fractionated plankton samples were collected monthly between April 2008 and January 2009 in the tidal reaches of the Skidaway River (Georgia, USA). Each size fraction, along with filtered (<30 μm) and bulk estuarine water, was processed according to U.S. Environmental Protection Agency method 1600. Presumptive enterococci were selected and species were identified using carbon substrate utilization patterns. The highest average densities occurred within the 30-, 63-, 105-, and 150-μm size fractions, which also represented the majority (>99%) of the particles within the sampled water. Particle-associated enterococci accounted for as little as 1% of enterococci in bulk water in April to as much as 95% in July. Enterococcus faecalis was the most commonly isolated species from both water and plankton and represented 31% (16/51) and 35% (6/17) of the identified Enterococcus species from water and plankton, respectively. Enterococcus casseliflavus represented 29% of the selected isolates from plankton and 16% from water. Both E. faecalis and E. casseliflavus were able to survive and grow in plankton suspensions significantly longer than in artificial seawater. Enterococcus spp. may be highly concentrated in plankton and associated particles, especially during summer and fall months. These findings could have implications for the effectiveness of enterococci as an indicator of coastal water quality, especially in particle-rich environments.

Fig 1

Water and plankton samples were collected between April 2008 and January 2009 at a fixed station on Skidaway Island (GA) along the Intracoastal Waterway (★).

Fig 2

(Top) Mean enterococcal densities in plankton fractions and bulk water (n = 2 for each date or fraction). Enterococcal levels in each plankton fraction were calculated by dividing the abundance in the concentrated fraction by the overall concentration factor for each size fraction to provide the equivalent enterococcal load in bulk water. (Bottom) Contribution of enterococci associated with each plankton fraction to the bulk water sample. Bars are additive for all plankton, and the relative contribution of each fraction is shown.

Fig 3

Abundance of plankton (diatoms and copepod nauplii), detritus, and total particles among each of the eight sampled size fractions for each sample date.

Fig 4

Growth and decay trends for enterococci in seawater and plankton microcosms. (a) E. faecalis at 10°C; (b) E. faecalis at 30°C; (c) E. casseliflavus at 10°C; (d) E. casseliflavus at 30°C. Arrows indicate day 22, when the contents of all microcosm containers were homogenized.