Zostera Marina Populations Research Articles

Characterization of disjunct populations of Zostera marina (eelgrass) from California: genetic differences resolved by restriction-fragment length polymorphisms

Comparative restriction-fragment analysis was used to analyze the nuclear ribosomal DNA, and alcohol dehydrogenase-1 loci of Zostera marina L., for variation within and among populations. Eelgrass is a perennial marine flowering plant that is widespread and ecologically significant throughout the temperate northern hemisphere. A chemical method was developed to obtain restriction-quality DNA without CsCl fractionation from experimentally relevant quantities of seagrass tissues (0.5 to 1.0 g). The yield was 25 μg g-1 fresh weight. The three morphologically distinct forms of Z. marina from disjunct populations examined in this study were found to be genetically distinct; morphologically similar populations were indistinguishable genetically. Genetic distinction also correlated with habitat depth, as subtidal and intertidal populations were clearly divergent. Homologous probes for the 17S and 28S ribosomal DNA genes were used to map 24 restriction sites on the rDNA repeat of Z. marina, which was determined to be about 14 kb in length. At least 1 length mutation and 5 restriction-site changes were identified that distinguished Z. marina populations from San Diego and Monterey Bay (Del Monte Beach) from Z. marina populations from Elkhorn Slough and Tomales Bay. Estimated sequence variation (100×p) between eelgrass populations ranged from 0.00 to 0.69. Individual plants were observed to contain as many as four different rDNA-repeat length variants. The mean number of rDNA-repeat length variants per individual in Z. marina was about two. Intrapopulation variation in rDNA-repeat type was observed in only one individual from the Tomales Bay population.

Nitrogen cycling in sediments with estuarine populations of Potamogeton perfoliatus and Zostera marina

Rates of nitrogen transformations and concentrations of extractable NH,+ and NOs(plus NO2-) were measured in estuarine sediments vegetated with the submersed macrophytes Potarnogeton perfoliatus and Zostera manna, and in adjacent bare sediments, 3 or 4 times during the growing season. Nitrification and denitrification potentials were measured in substrate-amended sediment slurries at 5 depth intervals to provide a measure of bacterial activity. In general, rates were significantly higher in vegetated compared to bare sediments. It appears that both plant species affected nitrogen transformations through several similar mechanisms, while the microbial community, in turn, regulated nitrogen available for plant growth. In P, perfoliatus beds, ammonification and potential nitrification rates were correlated. Both exhibited summer maxima coinciding with peak plant biomass and productivity. Although vertically integrated (0-12 cm) amrnonification rates were about t w c e as high in vegetated than in bare sediments, NH4+ pools were significantly lower, probably due to high plant nitrogen demand. In contrast, denitrification rates were highest in spring when NO3concentrations peaked, and were significantly correlated to nitrification rates in both spring and fall. Denitrification was only about 20% of total NO3reduction, suggesting that NH4+ production from NO3may be important in conserving nitrogen within the grassbed. In sediments with Z. marina, rates of ammonification, and nitrification and denitrificabon potentials each exhibited a distinct seasonal cycle, indicating that rates were not as tightly coupled as in P. perfoljatus beds. High amrnonification rates exceeded plant demand leading to NH,' accumulation. Potential nitrification rates were highest in vegetated sediments during fall. Denitrification rates, which were also greater in vegetated than in bare sediments, were highest in spring when NO3concentrations were high. Potential denitrification rates comprised about 10 of total NO3reduction, indicating that NO,reduction to NH,+ dominated. The microbial communities responsible for key nitrogen transformations in the sediments were enhanced by both P. perfoliatus and Z. manna ammonificatlon by inputs of organic nitrogen; nitrification by release of O2 by plant roots; and denitnfication by production of NO3-

“Wasting disease” and other dynamic phenomena in Zostera beds

The greatest decline of seagrass beds that is known is certainly the almost simultaneous breakdown of the North Atlantic populations of Zostera marina L., due to the “wasting disease”. Several explanations have been presented for this ecological disaster; however, none of those is satisfactory. It appears that for several localities a local explanation can be given. Apart from this general decline, it appears that, at least in Western Europe, there is a great variation in the temporal and spatial development cycles of the intertidal Zostera-beds. These variations can be ascribed to a number of factors, such as winter temperature (frost or no frost), grazing by birds (grazing or no grazing) and the balance between sedimentation (including sanding) and erosion. The combination “frost-grazing” leads to an annual cycle of the biomass, and a homogeneous bed structure. The combination “no frost-no grazing” causes raising of the seagrass bed, due to sedimentation up to a level where the environmental conditions become less favourable to seagrass; then breakdown sets in. These beds are heterogeneous and show a pluriannual developmental cycle.

Changes in the seagrass populations of the Dutch Waddenzee

A survey is given of the changes in the seagrass populations in the Dutch Waddenzee. A comparison of maps of the distribution in 1860 and 1930 shows that considerable changes took place before the “wasting disease” destroyed the sublittoral Zostera marina populations in the Waddenzee. These changes have to be regarded as normal long-term fluctuations within the large-scale pattern of the dynamic equilibrium of the Waddenzee ecosystem. In 1932 the “wasting disease” reached The Netherlands. The eulittoral populations of Z. marina did not succumb to it, while the eulittoral Z. noltii was not affected at all. In the period between 1932 and 1965 some quantitative fluctuations were noticed in the eulittoral populations of Z. marina and Z. noltii. After 1965 a general decline of both Zostera species commenced and is still in progress. This decline is no doubt a consequence of the increasing pollution, but the responsible factor has still not been ascertained beyond doubt.