The effects of global climate change on seagrasses

Effects of irradiance, temperature, and nutrients on growth dynamics of seagrasses: A review

In this paper, we explicitly link changes in community structure of estuarine primary pro- ducers to measured nitrogen loading rates from watersheds to estuaries, and quantify the relationship between nitrogen load, annual dynamics of algal growth and Zostera marina L productivity, and over- all eelgrass decline at the watershed-estuarine scale in estuaries of Waquoit Bay, Massachusetts, USA Substantial eelgrass loss (80 to 96% of bed area lost in the last decade) was found at loads of ~30 kgN ha -1 yr -1 , and total disappearance at loads ≥ 60 kg N ha -1 yr -1  Rather than decreased eelgrass growth rates, we observed an exponential decrease in shoot densities and bed area (and subsequently, areal pro- duction) as nitrogen loads increased, suggesting that eelgrass decline in higher-nitrogen estuaries of the Waquoit system occurred largely via lack of recruitment or enhanced mortality of established shoots Similar to the patterns observed in many other systems and the experimental results obtained in labo- ratories or mesocosms, the relationship we observed between nitrogen loads and eelgrass health within the Waquoit system was indirect: increased nitrogen stimulated growth and standing stocks of algal pro- ducers, that may have caused severe light limitation of eelgrass From light budgets that considered water column, epiphyte, and macroalgal shading, we estimated chronic, severe light limitation to newly recruiting shoots in higher-nitrogen estuaries, due mainly to shading by a coexisting ≤15 cm macro- algal canopy Two management recommendations aimed at eelgrass preservation emerge from this work First, development and management of watersheds must be conducted such that land-derived nitrogen loading to estuaries is restricted In the Waquoit Bay estuaries, for example, eelgrass is absent or rapidly disappearing from all but those receiving the lowest (≤15th percentile) loads Second, shoot density and meadow area, rather than growth rates per shoot, seem to be adequate variables for routine monitoring of eelgrass health We also show that the shift from eelgrass- to algae-dominated commu- nities has important consequences for total system primary production and carbon and nitrogen cycling Estimated total primary production by coastal assemblages in the Waquoit Bay system was 135% higher in estuaries receiving relatively high versus low loads of land-derived nitrogen, suggesting important trophic and biogeochemical alterations to temperate estuarine ecosystems as a result of eutrophication

https://www.int-res.com/articles/meps2003/247/m247p059.pdf

Eelgrass Zostera marina loss in temperate estuaries: relationship to land-derived nitrogen loads and effect of light limitation imposed by algae

The independent and interactive effects of nutrient concentration and epiphyte grazers on epiphyte biomass and macrophyte growth and production were examined in Zostera marina L. (eelgrass) microcosms. Experiments were conducted during early summer, late summer, fall, and spring in a greenhouse on the York River estuary of Chesapeake Bay. Nutrient treatments consisted of ambient or enriched (3× ambient) concentrations of inorganic nitrogen (ammonium nitrate) and phosphate. Grazer treatments consisted of the presence or absence of field densities of isopods, amphipods, and gastropods. epiphyte biomass increased with both grazer removal and nutrient enrichment during summer and spring experiments. The effect of grazers was stronger than that of nutrients. There was little epiphyte response to treatment during the fall, a result possibly of high ambient nutrient concentrations and low grazing pressure. Under low grazer densities of early summer, macrophyte production (g m−2 d−1) was reduced by grazer removal and nutrient enrichment independently. Under high grazer densities of late summer, macrophyte production was reduced by enrichment only with grazers absent. During spring and fall there were no macrophyte responses to treatment. The relative influence of epiphytes on macrophyte production may have been related to seasonally changing water temperature and macrophyte requirements for light and inorganic carbon.

/pdf/relative-effects-of-nutrient-enrichment-and-grazing-on-1ynq5coyh1.pdf

Relative effects of nutrient enrichment and grazing on epiphyte-macrophyte (Zostera marina L.) dynamics

Although the growth of eelgrass (Zostera marina) is controlled by resources as well as higher order interactions with epiphytes and their herbivores, these constraints rarely are considered together. The ability to utilize both water column and sediment nutrient sources in a complex habitat may provide eelgrass with a partial release from nutrient competition with epiphytes that have more efficient uptake kinetics and can reduce eelgrass growth, particularly in eutrophic habitats. We investigated the relative effects of dissolved inorganic nitrogen in the water column vs. the sediments, and herbivory by the common isopod Idotea resecata, on eelgrass growth and epiphyte biomass in an intertidal eelgrass bed in Padilla Bay, Washington. In the field, we fertilized the sediments and/or the water column with ammonium and measured eelgrass growth and epiphyte biomass. We also monitored epiphyte biomass and water column nutrient concentrations and cen- sused isopod densities. Laboratory experiments focused on the effects of I. resecata, fer- tilization of the water column and sediments, and depletion of sediment nutrients on eelgrass growth and epiphyte biomass. Most simply, we hypothesized that epiphytes would respond positively to increased water column nutrients as eelgrass would to increased sediment nutrients, and that herbivory on epiphytes could mitigate deleterious effects of epiphytes on eelgrass. We demonstrated that eelgrass growth is affected both by sediment nitrogen resources and the higher order effects of epiphytes and their control by Idotea resecata. During our field experiments, growth of eelgrass leaves tended to increase in response to sediment

James A. Marsh

Papers

Effects of temperature on photosynthesis and respiration in eelgrass (Zostera marina L.)