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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.

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This paper examines the character of phytoplankton blooms in a restricted sub-tropical lagoon along the Atlantic coast of central Florida. The results of the 23-year study (1997–2020) provide evidence for multiple types of variability in bloom activity, including cyclical patterns, stochastic events, and most prominently a regime shift in composition and intensity. Cyclical patterns (e.g., El Niño/La Niña periods) and stochastic events (e.g., tropical storms) influenced rainfall levels, which in turn impacted nutrient concentrations in the water column and the timing and intensity of blooms. In 2011, a major change occurred in the character of blooms, with a dramatic increase in peak biomass levels of blooms and the appearance of new dominant taxa, including the brown tide species Aureoumbra lagunensis and other nanoplanktonic species. Results of quantitative analyses reveal system behavior indicative of a regime shift. The shift coincided with widespread losses of seagrass community and reduced drift algae biomass. A combination of exceptionally low water temperatures in the winters of 2009/2010 and 2010/2011, hypersaline conditions associated with drought conditions, and high light attenuation caused by blooms appear to have contributed to the widespread and protracted decline in seagrass and drift macroalgal communities in the lagoon, leading to shifts in distribution of internal and external nutrient sources toward phytoplankton.

Seagrass is a major structural habitat in the Indian River Lagoon. Maps documented locations and areal extents of beds periodically since the 1940s, and surveys of fixed transects yielded changes in percent cover and depths at the end of the canopy since 1994. Areal extent increased by ∼7,000 ha from 1994 to 2009, mean percent cover within beds decreased from ∼40 to 20%, and mean percent cover standardized to maximum transect length remained near 20%. Thus, conditions supported a consistent biomass because cover decreased as areal extent increased. Between 2011 and 2019, ∼19,000 ha or ∼58% of seagrasses were lost, with offshore ends of canopies moving shoreward and shallower, and standardized mean percent cover decreased to ∼4%. These changes coincided with blooms of phytoplankton, and ≤ 27% of incident subsurface irradiance at 0.9 m was stressful. Decreases in mean percent cover per month of stress became larger when initial mean cover per transect was < 20%, which suggested that the ratio of aboveground to belowground tissues in the expanded and sparser beds led to respiratory demand that was not met by photosynthesis. Despite intermittent improvements in light penetration, widespread recovery of seagrasses has not occurred potentially due to detrimental feedbacks. For example, loss of seagrass exposed sediments to waves, and the resulting disturbance may have hampered recruitment of new shoots. The same decreases also made 58–88% of the carbon, nitrogen, and phosphorus in seagrass tissue available to other primary producers. These nutrients did not enhance growth of epiphytes, whose biomass decreased by ∼42%, but they apparently fueled blooms of phytoplankton, with mean chlorophyll-a concentrations increasing by > 900%. Such intense blooms increased shading and loss of seagrasses. Fortunately, data showed that patches of seagrasses at depths of 0.5–0.9 m persisted for 22–24 years, which suggested that this depth zone could hold the key to recovery. Nevertheless, optimistic estimates predict recovery could take 12–17 years. Such a long-term, widespread loss of a key structural habitat may generate multiple adverse effects in the system, and mitigating such effects may entail planting seagrasses to accelerate recovery.

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