Fragments of the seagrasses Halodule wrightii and Halophila johnsonii as potential recruits in Indian River Lagoon, Florida

Abstract

The role of vegetative seagrass fragments as a dispersal and recruitment mechanism has received little attention. Research on the potential of vegetative fragments as a dispersal mechanism can help us better understand the ability of seagrass beds to recover from disturbance events, to recruit into new areas, and to survive over long periods. The objectives of this study were to (1) determine the viability of vegetative fragments of Halodule wrightii and Halophila johnsonii as a function of time after removal from sediment, (2) determine whether season of collection affects the fragments' recruitment potential, (3) determine if the source of fragments of H. johnsonii affects viability, (4) determine how long fragments float, and (5) determine the frequency of fragment settlement and rooting vs. time. Mesocosm experiments with plants collected from Indian River Lagoon, Florida demonstrated that fragments of H. wrightii remain viable during spring for up to 4 wk with a marked decline in survival after 2 wk of drifting. Fall plants had a shorter period of viability with only 5% of fragments remaining viable by Week 2. Although the source location of the fragments did not influence viability for H. johnsonii, day and season were highly significant for viability, with spring plants remaining viable for up to 4 d and fall plants remaining viable for twice as long. The short viability of H. johnsonii illustrates the importance of rapid settlement when uprooted from a source bed, limiting dispersal to short distances. H. wrightii appears to maintain its viability for a longer period of time, indicating that this species may be able to utilize fragments as a dispersal mechanism over longer distances. The vegetative fragments of both H. wrightii and H. johnsonii had the ability to settle and root in mesocosms, demonstrating that fragmentation is a viable mechanism for dispersal and recruitment for these species.

Expansion and fragment settlement of the non-native seagrass Halophila stipulacea in a Caribbean bay

“…This short-term rooting of vegetative H. stipulacea fragments from the first field experiments are in line with ex situ experiments where H. wrightii and H. johnsonii fragments settled on the sediment and started rooting within two weeks (Hall et al 2006). In another experiment from the same study where settlement was prevented, individual H. wrightii fragments remained viable for four weeks after being fragmented, and H. johnsonii for eight days at most (Hall et al 2006). During this time the fragments can travel and reach new grounds to colonize.…”

“…Given our average production of 0.9 shoots per day, Caribbean H. stipulacea appears to grow remarkably faster, possibly due to the high temperatures and light availability in shallow Caribbean bays (Debrot et al 2012). This short-term rooting of vegetative H. stipulacea fragments from the first field experiments are in line with ex situ experiments where H. wrightii and H. johnsonii fragments settled on the sediment and started rooting within two weeks (Hall et al 2006). In another experiment from the same study where settlement was prevented, individual H. wrightii fragments remained viable for four weeks after being fragmented, and H. johnsonii for eight days at most (Hall et al 2006).…”

“…Our first quantification of properties of introduced seagrass fragments calls for further investigation of fragment characteristics and interspecies competition in order to determine the invasiveness of H. stipulacea and be able to model non-native seagrass expansion. To fill in the most important gaps of knowledge around H. stipulacea invasiveness potential and expansion, more knowledge is needed on fragment properties (viability, floating duration, dispersal distance, uprooting factors, density, lifespan; Ceccherelli & Cinelli 1999;Smith & Walters 1999;Hall et al 2006;Grech et al 2016) and the impact of natural and human-induced stressors (e.g. grazing, storms, anchor or propeller damage; Bando 2006;West et al 2007West et al , 2009.…”

“…Fragments of colonizing seagrass species, i.e. H. wrightii and Halophila johnsonii N.J.Eiseman have been reported to settle and root within two weeks in mesocosms (Hall et al 2006). So far, in situ studies on the potential of vegetative fragments as a dispersal mechanism for the non-native seagrass H. stipulacea are lacking.…”

Seed addition facilitates eelgrass recovery in a coastal bay system

“…These natural recruitment processes can be important for bed persistence, especially after diebacks due to stressful events such as unusually high summertime temperatures (Jarvis & Moore 2010), high turbidity events due to phytoplankton blooms (Dennison et al 1989), or storms (Williams 1988). We have discounted dispersal and establishment of vegetative fragments in developing new patches, as there is little evidence that these fragments can successfully establish (Ewanchuk & Williams 1996, Di Carlo et al 2005, Hall et al 2006. The relatively low rates of seedling establishment at 6 mo (medians of 2 to 7% of viable seeds dis tributed) in our unvegetated seed plots are largely a result of wintertime losses of early-stage, shallow seedlings which, without a substantial root system, are easily removed by wave action and currents that can erode sediments to several centimeters (Ma rion & Orth 2012, this Theme Section).…”