W. Judson Kenworthy

Accelerating loss of seagrasses across the globe threatens coastal ecosystems

Coastal ecosystems and the services they provide are adversely affected by a wide variety of human activities. In particular, seagrass meadows are negatively affected by impacts accruing from the billion or more people who live within 50 km of them. Seagrass meadows provide important ecosystem services, including an estimated

A Global Crisis for Seagrass Ecosystems

1.9 trillion per year in the form of nutrient cycling; an order of magnitude enhancement of coral reef fish productivity; a habitat for thousands of fish, bird, and invertebrate species; and a major food source for endangered dugong, manatee, and green turtle. Although individual impacts from coastal development, degraded water quality, and climate change have been documented, there has been no quantitative global assessment of seagrass loss until now. Our comprehensive global assessment of 215 studies found that seagrasses have been disappearing at a rate of 110 km2 yr−1 since 1980 and that 29% of the known areal extent has disappeared since seagrass areas were initially recorded in 1879. Furthermore, rates of decline have accelerated from a median of 0.9% yr−1 before 1940 to 7% yr−1 since 1990. Seagrass loss rates are comparable to those reported for mangroves, coral reefs, and tropical rainforests and place seagrass meadows among the most threatened ecosystems on earth.

The Effects of Long-Term Manipulation of Nutrient Supply on Competition between the Seagrasses Thalassia testudinum and Halodule wrightii in Florida Bay

ABSTRACT Seagrasses, marine flowering plants, have a long evolutionary history but are now challenged with rapid environmental changes as a result of coastal human population pressures. Seagrasses provide key ecological services, including organic carbon production and export, nutrient cycling, sediment stabilization, enhanced biodiversity, and trophic transfers to adjacent habitats in tropical and temperate regions. They also serve as “coastal canaries,” global biological sentinels of increasing anthropogenic influences in coastal ecosystems, with large-scale losses reported worldwide. Multiple stressors, including sediment and nutrient runoff, physical disturbance, invasive species, disease, commercial fishing practices, aquaculture, overgrazing, algal blooms, and global warming, cause seagrass declines at scales of square meters to hundreds of square kilometers. Reported seagrass losses have led to increased awareness of the need for seagrass protection, monitoring, management, and restoration. However, seagrass science, which has rapidly grown, is disconnected from public awareness of seagrasses, which has lagged behind awareness of other coastal ecosystems. There is a critical need for a targeted global conservation effort that includes a reduction of watershed nutrient and sediment inputs to seagrass habitats and a targeted educational program informing regulators and the public of the value of seagrass meadows.

Integrating biology and economics in seagrass restoration: How much is enough and why?

Long term (8 yr) continuous fertilization (via application of bird feces) of established seagrass beds in Florida Bay, FL, USA caused a change in the dominant seagrass species. Before fertilization, the seagrass beds were a Thalassia testudinum monoculture; after 8 yr of fertilization the seagrass Halodule wrightii made up 97% of the aboveground biomass. Fertilization had a positive effect on the standing crop of T. testudinum for the first two years of the experiment. The transition from T. testudinum-dominated to H. wrightii-dominated was dependent on the timing of colonization of the sites by H. wrightii; the decrease in T. testudinum standing crop and density at the fertilized sites occurred only after the colonization of the sites by H. wrightii. There were no trends in the standing crop or density of T. testudinum at control sites, and none of the control sites were colonized by H. wrightii. The effects of fertilization on these seagrass beds persisted at least 8 yr after the cessation of nutrient addition, suggesting that these systems retain and recycle acquired nutrients efficiently. Results of these experiments suggest that Halodule wrightii, the normal early-successional seagrass during secondary succession in Caribbean seagrass communities, has a higher nutrient demand than Thalassia testudinum, the normal late successional species, and that the replacement of H. wrightii by T. testudinum during secondary succession is due to the ability of T. testudinum to draw nutrient availability below the requirements of H. wrightii

Seagrass Conservation Biology: An Interdisciplinary Science for Protection of the Seagrass Biome

Abstract Although success criteria for seagrass restoration have been in place for some time, there has been little consistency regarding how much habitat should be restored for every unit area lost (the replacement ratio). Extant success criteria focus on persistence, area, and habitat quality (shoot density). These metrics, while conservative, remain largely accepted for the seagrass ecosystem. Computation of the replacement ratio using economic tools has recently been integrated with seagrass restoration and is based on the intrinsic recovery rate of the injured seagrass beds themselves as compared with the efficacy of the restoration itself. In this application, field surveys of injured seagrass beds in the Florida Keys National Marine Sanctuary (FKNMS) were conducted over several years and provide the basis for computing the intrinsic recovery rate and thus, the replacement ratio. This computation is performed using the Habitat Equivalency Analysis (HEA) and determines the lost on-site services pertaining to the ecological function of an area as the result of an injury and sets this against the difference between intrinsic recovery and recovery afforded by restoration. Joining empirical field data with economic theory has produced a reasonable and typically conservative means of determining the level of restoration and this has been fully supported in Federal Court rulings. Having clearly defined project goals allows application of the success criteria in a predictable, consistent, reasonable, and fair manner.

The use of fertilizer to enhance growth of transplanted seagrasses Zostera marina L. and Halodule wrightii Aschers

In the past three decades seagrass research has adopted several disciplines and matured into a global science. One of the approaches we can use to focus our science to benefit the management and protection of seagrass is that of Conservation Biology; a proactive field of science bringing together academic, government, and nongovernmental organizations from a wide range of disciplines to understand and conserve biodiversity. This relatively recent field synthesises and directs insights from many disciplines for direct application to the protection and conservation of species, communities, and biomes (Fig. 1). While the primary focus for conservation biology comes from ecology, genetics, landscape ecology, population biology and taxonomy, the discipline also incorporates analytical procedures associated with the social sciences, biogeography, and evolutionary biology (Soule and Wilcox, 1980; Soule, 1985; Meffe and Carroll, 1994; Primack, 2000). Conservation biology recognizes that humans derive both extractive and intrinsic benefit from the natural world and embraces methods and analyses utilized in fisheries science, agriculture, anthropology, economics, law, philosophy, and sociology. Today, unlike traditional approaches that were rooted in the preservation and management of selected species, conservation biologists are advising natural resource managers to focus more on an ecosystem approach that includes entire biomes, and to recognize that public trust demands comprehensive protection of biodiversity as much as sustaining the yield of harvestable organisms. Conservation biology endeavors to maintain and protect biodiversity at all spatial scales, including a variety of little understood and often overlooked life forms. In the broader meaning of biodiversity we are interested in conserving ecological services as much as life forms (sensu Randall, 1986).

Evaluation of aboveground and belowground biomass recovery in physically disturbed seagrass beds.

Abstract The effect of two slow release fertilizers on the survival and growth of transplants of two seagrasses, Zostera marina L. and Halodule wrightii Aschers was examined. The two fertilizers, an unbalanced nitrogen, phosphorus, and potassium formulation (18-0-0) and a balanced (14-14-14) formulation, were applied to bare root transplant units (PU) of each seagrass at three doses, 10, 90, and 170 g per PU. Survival and growth of the transplants, nutrient release from the fertilizers, and several environmental characteristics of the study sites were examined. Nitrogen enrichment vegetative reproduction, the rate of area covered and leaf growth in a fall transplant of Z. marina , but only moderately stimulated growth in the spring, confirming that nitrogen may limit the rate of development of newly established populations of Z. marina . There was no effect of nitrogen on survival of Z. marina nor on the growth and survival of H. wrightit transplants. No phosphorus was released from the balanced fertilizers in the spring and fall experiments and no nitrogen was released from the unbalanced formulation in the spring Z. marina experiment. Therefore, it was not possible to examine the effect of nitrogen and phosphorus interactions on plant growth and vegetative reproduction. Nitrogen enrichment may be used to stimulate shoot growth of Z. marina transplants, and we suggest alternative procedures of fertilizer application to overcome problems of nutrient release.

Seagrasses in the age of sea turtle conservation and shark overfishing

Several studies addressed aboveground biomass recovery in tropical and subtropical seagrass systems following physical disturbance. However, there are few studies documenting belowground biomass recovery despite the important functional and ecological role of roots and rhizomes for seagrass ecosystems. In this study, we compared the recovery of biomass (g dry weight m−2) as well as the biomass recovery rates in ten severely disturbed multi-species seagrass meadows, after the sediments were excavated and the seagrasses removed. Three sites were located in the tropics (Puerto Rico) and seven in the subtropics (Florida Keys), and all were originally dominated by Thalassia testudinum. Total aboveground biomass reached reference values at four out of ten sites studied, two in the Florida Keys and two in Puerto Rico. Total belowground biomass was lower at the disturbed locations compared to the references at all sites, apart from two sites in the Florida Keys where the compensatory effect of opportunistic species (Syringodium filiforme and Halodule wrightii) was observed. The results revealed large variation among sites in aboveground and belowground biomass for all species, with higher aboveground recovery than belowground for T. testudinum. Recovery rates for T. testudinum were highly variable across sites, but a general trend of faster aboveground than belowground recovery was observed. Equal rates between aboveground and belowground biomass were found for opportunistic species at several sites in the Florida Keys. These results indicate the importance of belowground biomass when assessing seagrass recovery and suggest that the appropriate metric to assess seagrass recovery should address belowground biomass as well as aboveground biomass in order to evaluate the full recovery of ecological services and functions performed by seagrasses. We point out regional differences in species composition and species shifts following severe disturbance events and discuss ecological implications of gap dynamics in multi-species seagrass meadows.

Fluorescence census techniques for coral recruits

Efforts to conserve globally declining herbivorous green sea turtles have resulted in promising growth of some populations. These trends could significantly impact critical ecosystem services provided by seagrass meadows on which turtles feed. Expanding turtle populations could improve seagrass ecosystem health by removing seagrass biomass and preventing of the formation of sediment anoxia. However, overfishing of large sharks, the primary green turtle predators, could facilitate turtle populations growing beyond historical sizes and trigger detrimental ecosystem impacts mirroring those on land when top predators were extirpated. Experimental data from multiple ocean basins suggest that increasing turtle populations can negatively impact seagrasses, including triggering virtual ecosystem collapse. Impacts of large turtle populations on seagrasses are reduced in the presence of intact shark populations. Healthy populations of sharks and turtles, therefore, are likely vital to restoring or maintaining seagrass ecosystem structure, function, and their value in supporting fisheries and as a carbon sink.

Flowering and genetic banding patterns of Halophila johnsonii and conspecifics

Recruitment processes influence coral population dynamics as well as reef community structure. Coral recruitment is generally determined by one of three methods: artificial settlement plates (e.g. Rogers et al. 1984; Harriott and Fisk 1987; Mundy 2000), small-scale macro photography (Smith 1997; Edmunds et al. 1998), or painstaking visual searches in the field (Edmunds et al. 1998; Miller et al. 2000). These methods can be labor-intensive or time-consuming, as they require microscopic examination of the settlement surface or enough time for the coral to grow large enough to be visible to the naked eye. For example, coral recruits on settlement plates may be 0.7–15 mm in diameter (Jaap et al. 1994). Field survey techniques generally consider corals smaller than 5 cm diameter (2 cm for small species) to be juveniles (Chiappone and Sullivan 1996; Miller et al. 2000), while recruits have size classes of 0.1–2 cm diameter (Mumby 1999; Miller et al. 2000). Corals and their symbiotic zooxanthellae both contain fluorescent pigments that absorb light at a certain wavelength and re-emit it at a different wavelength. Chlorophyll in the zooxanthellae harvests light for photosynthesis and fluoresces red at about 685 nm. The coral host also contains fluorescent pigments, which could provide photoprotection (Salih et al. 2000; Vermeij et al. 2002) or amplify the light available for photosynthesis (Schlichter et al. 1999; Vermeij et al. 2002); however, other research suggests fluorescent proteins have no effect on photon absorption and have no photosynthetic function (Mazel et al. 2003). Host pigments typically exhibit fluorescence peaks between 480 nm and 590 nm (Mazel 1995; Myers et al. 1999;Manica andCarter 2000).When excited with blue or ultraviolet light, corals often fluoresce green or orange (Logan et al. 1990; Mazel 1995; Vermeij et al. 2002). This distinguishes corals from other chlorophyllcontaining reef organisms such as algae, which appear red. Fluorescence is a relatively weak effect and is difficult to observe during the day, but is easily seen at night because of increased contrast and reduced backscatter. Recent advances in the use of fluorescence imagery allow photographic detection of reef invertebrates as small as 1 mm diameter (C. Mazel, personal communication). We evaluated the capability of fluorescence technologies to identify and enumerate scleractinian coral recruits, and to compare this technique with current methods used to quantify coral recruitment. The working hypothesis was that the increased contrast and reduced backscatter of fluorescence techniques would facilitate the observation of coral recruits. If fluorescence can be developed as a rigorous tool to count coral recruits, it can potentially be a simple, sensitive tool for rapid estimates of the distribution and abundance of scleractinian recruits at a variety of spatial scales. This could be a valuable tool to guide coral reef ecosystem management; for example, fluorescence could provide rapid, non-invasive, site-specific estimates coral recruits needed as input data for spatially explicit coral recovery models (Whitfield et al. 2001).