Pollen limitation may be a common Allee effect in marine hydrophilous plants: implications for decline and recovery in seagrasses.

Pollen limitation may be an important factor in accelerated decline of sparse or fragmented populations. Little is known whether hydrophilous plants (pollen transport by water) suffer from an Allee effect due to pollen limitation or not. Hydrophilous pollination is a typical trait of marine angiosperms or seagrasses. Although seagrass flowers usually have high pollen production, floral densities are highly variable. We evaluated pollen limitation for intertidal populations of the seagrass Zostera noltei in The Netherlands and found a significant positive relation between flowering spathe density and fruit-set, which was suboptimal at <1200 flowering spathes m(-2) (corresponding to <600 reproductive shoots m(-2)). A fragmented population had ≈35 % lower fruit-set at similar reproductive density than a continuous population. 75 % of all European populations studied over a large latitudinal gradient had flowering spathe densities below that required for optimal fruit-set, particularly in Southern countries. Literature review of the reproductive output of hydrophilous pollinated plants revealed that seed- or fruit-set of marine hydrophilous plants is generally low, as compared to hydrophilous freshwater and wind-pollinated plants. We conclude that pollen limitation as found in Z. noltei may be a common Allee effect for seagrasses, potentially accelerating decline and impairing recovery even after environmental conditions have improved substantially.

Factors influencing primary production of seagrass beds (Zostera noltii Hornem.) in the Thau lagoon (French Mediterranean coast).

The primary production and the respiration of Zostera noltii beds in the Thau lagoon were studied by means of the benthic bell jar technique. Concurrently, environmental data (temperature, light and nutrients) as well as morphological data of seagrass meadows (leaf width and height, density of shoots, above/below-ground biomass ratio) were collected with the purpose of explaining most of the observed variations in metabolism. Seagrass plus epiphyte respiration rates were influenced mainly by the water temperature, showing a typical exponential response to an increase in temperature. Surprisingly, measurements of production rates were not related to incoming light intensities recorded at the seagrass canopy level. An equation frequently used for terrestrial standing crops, involving the leaf area index (LAI) and the characteristics of the canopy architecture (parameter K, depending on leaves optical and geometrical properties), was applied to the seagrass ecosystem in order to estimate the light energy actually available for the plants, i.e. the light intercepted by the seagrass canopy (Q(abs)). Linear relationships were then validated between gross production rates and calculated Q(abs) for Z. noltii beds, and the best fits were obtained with K values nearing 0.6, confirming the similarities between terrestrial graminaceae and seagrasses. A linear regression model for primary production is proposed, involving the calculated Q(abs), the water temperature and the leaf nutrient content.