Cultivation grazing response in seagrass may depend on phosphorus availability

Holzer, Kimberly K.; McGlathery, Karen J.

doi:10.1007/s00227-016-2855-5

Cited by 23 publications

(29 citation statements)

References 45 publications

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“…We note that these increases in mass‐specific productivity are not exclusively driven by absolute increases in new growth, but rather by changes in relative growth (i.e. shifts in the amount of newly produced leaf material relative to older seagrass biomass; Holzer & McGlathery, ). Mechanisms of grazer‐stimulated productivity have been previously documented in seagrasses (Kuiper‐Linley, Johnson, & Lanyon, ; Moran & Bjorndal, ; Valentine, Heck, Busby, & Webb, ), and may result from an increased reallocation of stored carbohydrates towards leaf growth to replace lost tissue.…”

Section: Discussionmentioning

confidence: 93%

Herbivore community determines the magnitude and mechanism of nutrient effects on subtropical and tropical seagrasses

et al. 2017

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Abstract1. Numerous studies have examined the combined effects of nutrients (bottom-up control) and consumers (top-down control) on ecosystem structure and functioning. While it is recognized that both can have important effects, there remains a limited understanding of how their relative importance shifts across large spatial scales where consumer functional types can vary.2. Using seagrasses as a model ecosystem, we explore the effects of ambient variation in herbivore functional composition on the relative importance of bottom-up and top-down forcings. Distributed experiments were conducted across four Western Atlantic sites that encompassed a gradient in consumer composition. Herbivores at two subtropical sites were predominantly comprised of small crustacean invertebrates (mesograzers that consume epiphytic algae), while herbivores at two tropical sites were dominated by large macrograzers (sea urchins and herbivorous fishes that directly consume seagrass biomass).3. To test the relative importance of bottom-up and top-down factors at each site, we manipulated nutrient supply, mesograzer abundance, and the presence of macrograzers (mainly herbivorous fishes) in a fully factorial design over a 14-week growing season. Seagrass above-ground biomass, shoot density, canopy cover, leaf productivity and epiphyte mass were measured as indicators of habitat structure and productivity.4. Overall, nutrient addition elicited a strong response across sites, causing widespread loss of seagrass biomass and shoot density. However, the mechanisms driving these declines strongly varied as a function of resident herbivore identity.Seagrass loss at tropical sites was attributable to macrograzers, which increased their direct consumption of fertilized, nutrient-rich seagrass. Conversely, at the subtropical sites, nutrient loading caused seagrass declines associated with the proliferation of epiphytic algae, but only in locations where mesograzer abundance was low.

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Section: Discussionmentioning

confidence: 93%

Herbivore community determines the magnitude and mechanism of nutrient effects on subtropical and tropical seagrasses

et al. 2017

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“…Prior to any measurements, the seagrass rhizomes were severed around the edges of all plots using a flat-bladed shovel to prevent the translocation of nutrients into the experimental plots from the surrounding unclipped meadow 10 . This was done to simulate a grazed area larger than 2 × 2 m, which is at the smaller end of the size range of natural grazing plots 44 – 47 . Reference plots were also severed to ensure that any effects seen in the clipped plots were the result of clipping, and not from severing rhizomes.…”

Section: Methodsmentioning

confidence: 99%

Blue carbon stores in tropical seagrass meadows maintained under green turtle grazing

Johnson

Gulick

Bolten

et al. 2017

Sci Rep

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Seagrass meadows are important sites for carbon storage. Green turtles (Chelonia mydas) are marine megaherbivores that consume seagrass throughout much of their global range. With successful conservation efforts, turtle abundance will increase, leading to more meadows being returned to their natural grazed state. There is concern this may lead to a loss of carbon stored in these systems, but the effects of green turtle grazing on seagrass ecosystem carbon dynamics have not been investigated. Here we experimentally show that despite 79% lower net ecosystem production (NEP) following grazing (24.7 vs. 119.5 mmol C m−2 d−1) in a Caribbean Thalassia testudinum seagrass meadow, grazed areas maintained net positive metabolic carbon uptake. Additionally, grazing did not change the meadow production to respiration ratio, indicating it did not stimulate remineralization of sediment carbon stores. Compared to other published estimates of seagrass NEP (median: 20.6 mmol C m−2 d−1), NEP in grazed Caribbean T. testudinum meadows is similar to that in many other ungrazed systems. Our results demonstrate that while grazing does decrease potential future carbon sequestration as a result of lower NEP, it does not promote a metabolic release of current carbon stocks.

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“…Phosphorus (P) content instead did not change among grazing levels. Holzer & McGlathery (2016) studied responses of T. testudinum to grazing in a phosphorus limited environment in Bermuda in plots with and without artificial fertilizers; and found that the cultivation grazing response of the turtles depended on the availability of phosphorus. In carbonate sediments phosphorus (P) is often the growth-limiting nutrient of many tropical seagrasses (Fourqurean et al, 1995).…”

Section: Discussionmentioning

confidence: 99%

“…In terrestrial and marine systems, the impact of grazing on plants results from both top-down (grazing) and bottom-up effects (nutrients; De Mazancourt, Loreau & Abbadie, 1998; Holzer & McGlathery, 2016). Ammonium concentrations in pore-water sediments showed the inverse tendency to T. testudinum density and biomass; which decreased when grazing pressure increased.…”

Section: Discussionmentioning

confidence: 99%

Nutrient availability induces community shifts in seagrass meadows grazed by turtles

López

Akker

Walk

et al. 2019

PeerJ

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In the Caribbean, green turtles graze seagrass meadows dominated by Thalassia testudinum through rotational grazing, resulting in the creation of grazed and recovering (abandoned) patches surrounded by ungrazed seagrasses. We evaluated the seagrass community and its environment along a turtle grazing gradient; with the duration of (simulated) grazing as a proxy for the level of grazing pressure. The grazing levels consisted of Short-term (4 months clipping), Medium-term (8 months clipping), Long-term grazing (8 months of clipping in previously grazed areas), 8-months recovery of previously grazed patches, and ungrazed or unclipped patches as controls. We measured biomass and density of the seagrasses and rhizophytic algae, and changes in sediment parameters. Medium- and Long-term grazing promoted a shift in community species composition. At increasing grazing pressure, the total biomass of T. testudinum declined, whereas that of early-successional increased. Ammonium concentrations were highest in the patches of Medium-term (9.2 + 0.8 μM) and Long-term grazing levels (11.0 + 2.2 μM) and were lowest in the control areas (4.6 + 1.5 μM). T. testudinum is a late-successional species that maintains sediment nutrient concentrations at levels below the requirements of early-successional species when dominant. When the abundance of this species declines due to grazing, these resources become available, likely driving a shift in community composition toward a higher abundance of early-successional species.

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Cultivation grazing response in seagrass may depend on phosphorus availability

Cited by 23 publications

References 45 publications

Herbivore community determines the magnitude and mechanism of nutrient effects on subtropical and tropical seagrasses

Herbivore community determines the magnitude and mechanism of nutrient effects on subtropical and tropical seagrasses

Blue carbon stores in tropical seagrass meadows maintained under green turtle grazing

Nutrient availability induces community shifts in seagrass meadows grazed by turtles

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