Marina Populations Research Articles

Effects of water dynamics on Zostera marina: transplantation experiments in the intertidal Dutch Wadden Sea

Pilot experiments in the tidally dominated Dutch Wadden Sea indicated a negative rela- tionship between Zostera marina L. transplantation success and tidal depths. As light availability was sufficient, we hypothesised that water dynamics (particularly waves) and ensuing sediment mobility (movement or resuspension of the sediment) were the major cause for the loss of transplants at larger depths. Transplantation experiments were carried out at intertidal flats and depths under conditions of normal and reduced water dynamics and sediment mobility, using exclosures that reduced water dynamics and shells that stabilised the sediment. To test bioturbation effects, cages that excluded large biota were used. Without protection, Z. marina plants were successfully established in a belt within the intertidal zone (0 to -0.20 m mean sea level, MSL) at 2 out of 3 transplantation sites during the first growing season. Reduced water dynamics by exclosures prevented the loss of plants in the zone -0.40 to -1.15 m MSL; removal of the exclosures after 1 mo resulted in loss of all plants within a few days. Neither light limitation nor bioturbation could explain these results. We conclude that prevalent water dynamics, particularly the relative period of exposure to wave dynamics, were too high for establishment and maintenance of intertidal Z. marina at depths below -0.20 m MSL at these sites of intermediate exposure. At the third, exposed, transplantation site, water dynamics prevented transplantation success along the entire depth gradient studied. Reduction of sediment mobility by shells had a positive effect on transplant survival, particularly at -0.20 m MSL. Shell armour can therefore be recommended in transplantations near the lower limit of potential intertidal habitats of Z. marina. Anchorage of the plants with pegs had no positive effect. Depth limitation of intertidal Z. marina populations by water dynamics can explain zonation patterns that occur in several tidal sys- tems in northwest Europe.

Temperature and salinity constraints on the life cycle of two brackish-water nematode species

Abstract The present study investigates the influence of salinity and temperature on the life history of two estuarine bacterivorous nematode species, Pellioditis marina and Diplolaimelloides meyli, isolated from the mesohaline zone of the Westerschelde Estuary, SW Netherlands. Gravid females and adult males were inoculated in petri dishes containing agar layers of nine (for P. marina) or five (for D. meyli) different salinities, from almost freshwater to higher than marine, and incubated at a temperature of 20°C, to study the impact of salinity; agar layers with a salinity of 20‰, incubated under each of six different temperatures from 5 to 30°C, served to study the effect of temperature. Daily and total fecundity, development time and sex ratio were quantified, and preadult mortality was estimated. The results are compared to those of a partner study on the influence of salinity and temperature on respiration, assimilation and scope for production in the same nematode species. Salinity had relatively minor effects on fecundity, development times and sex ratio in both species, but strongly impacted juvenile viability at the extremes of the salinity range: at salinities close to 0 and 40‰, preadult mortality was more than 80% in P. marina; it was 100% at 5‰ in D. meyli. Both species had an (near) optimal fitness at salinities of 10 to 30‰. Temperature had a pronounced influence on both nematodes over the entire range studied. Diplolaimelloides meyli still reproduced and matured at temperatures exceeding 30°C, while P. marina had an upper temperature limit for reproduction of 25°C. Development times of D. meyli were more temperature-dependent than those of P. marina: the mean development time from adult to adult for the latter nematode ranged from 2 days at 25°C to 7 days at 9°C. The development time of D. meyli increased from 7 days at 25–30°C to 63 days at 10°C, temperature below which no reproduction occurred. Female-biased sex ratios were found in D. meyli at low temperatures and in P. marina under optimal salinity conditions. The life history results largely agree with the predicted scope for production, but discrepancies were found near the extremes of the abiotic range of both species. It is emphasized that the ranges observed are characteristic of populations, not of species; they may to an extent have been influenced by culture conditions. A comparison of the present results with literature data on other P. marina populations demonstrates that some populations of this species may still reproduce successfully under conditions which are lethal to other populations, raising the question as to whether cryptic species rather than populations of a single species are involved.

Suitability of Zostera marina populations for transplantation to the Wadden Sea as determined by a mesocosm shading experiment

As part of eelgrass re-establishment efforts in the Wadden Sea, five northwest European eelgrass ( Zostera marina L.) populations were studied in a mesocosm filled with Wadden Sea water over a 2-year period. The original habitats of these populations varied with regard to salinity, tidal regime, latitude, substrate type and water nutrient content. The reproductive strategies ranged from semi-annual to perennial. Experimental parameters studied were: planting density (64 and 121 plants m −2) and irradiance corresponding to 75%, 89%, 95% and 98% light reduction at the bottom. Three out of five Z. marina populations were successfully transplanted. Planting density, although initially positively correlated, had no effect on final transplantation success. The populations maintained their original reproductive strategy. The reproductive strategy of the donor population did not show a clear correlation with transplantation success. The maximum light reduction tolerated at the bottom was 75%–89%. The populations did not differ in shading tolerance. Assuming a light attenuation coefficient in the Wadden Sea of k=2.0 m −1, this corresponds to a maximum transplantation depth of between 0.45 m and 0.85 m below mean sea level. The differences in transplantation success, and the conservative reproductive strategy of different Z. marina populations, indicate genetic differences and/or low phenotypic plasticity. Moreover, this implies that the characteristics of a donor population must be carefully considered prior to transplantation.

Estimates of outcrossing rates and of inbreeding depression in a population of the marine angiosperm Zostera marina

Despite the great diversity of pollination and fertilization mechanisms observed in marine plants, little is known about the causes or maintenance of this variation. In this study, I estimated outcrossing rates and levels of inbreeding depression in Zostera marina L. (eelgrass), providing the first empirical test of hypotheses about the evolution of breeding systems in plants with submerged flowers. This study also addressed temporal separation of female and male flowering (dichogamy) in eelgrass as a mechanism promoting an outcrossing mating system, and whether the mating system in eelgrass is related to the degree of dichogamy in the field. Outcrossing rates (0<t<1) estimated from two polymorphic allozyme loci indicate that the Z. marina population in intertidal and subtidal habitats in False Bay, Washington, USA, was highly outcrossing in both 1991 (intertidal t=0.905, subtidal t=1.0) and 1992 (intertidal t=0.775, subtidal t=0.611). The outcrossing rates were positively associated with the degree of dichogamy in 1992; intertidal plants exhibited a greater temporal separation of female and male flowering and a higher outcrossing rate than did subtidal plants. Inbreeding depression at seed set was estimated from hand pollinations (self- and outcross) on 20 reproductive individuals from the False Bay population. Averaged across all maternal parents, a greater proportion of outcrossed flowers set seed than selfed flowers; i.e., inbreeding depression was detected. Plants exhibited genetic variation for inbreeding depression, detected as a significant pollination treatment × maternal family interaction in a log-likelihood analysis. By the end of the seed-maturation period (7 mo after intial seed set) some families showed outbreeding depression, i.e., greater fitness in progeny derived from selfing than in progeny from outcrossing. The inbreeding depression in the False Bay population may be an important selective factor contributing to the maintenance of dichogamy and an outcrossing mating system, as proposed for aquatic plants.

Comparative study on populations of Zostera marina L. (eelgrass): in situ nitrogen enrichment and light manipulation

The combined effects of light and nitrogen availability on population dynamical characteristics (e.g. growth rate, biomass, density, flowering) of a perennial and semi-annual population of Zostera marina L. were experimentally studied in two non-tidal lagoons (Grevelingen and Veerse Meer). Light reduction ( ≈ 70 % ) significantly reduced all population dynamical features measured in both populations. Under reduced light uptake of nitrogen occurred by plants of the semi-annual population, but the nutrient could not be used for growth. After enrichment with nitrogen plants of neither population stored nitrogen; the nutrient was immediately used. Light limitation of growth remained undetermined for the semi-annual population, although mean values for biomass and density characteristics doubled at light enhancement (studied in the Veerse Meer only). Nitrogen enrichment had a significantly positive effect on the perennial population; leading to enhanced above-ground relative growth rate, flowering and above-ground biomass. A lower probability of recruitment through generative reproduction (flowering) of the perennial population caused by nitrogen limitation apparently does not become compensated for by a possible higher vegetative production of shoots. Nitrogen limitation is thought to be a concomitant factor in the sharp decline of Z. marina populations since 1985 in the Grevelingen.

Surface and submarine pollination in the seagrass Zostera marina L.

COX, P. A., LAUSHMAN, R. H. & RUCKLESHAUS, M. H., 1992. Surface and submarine pollination in the seagrass Zostera marina L. Hydrophilous plants can be divided into three ecological categories depending upon whether their pollen is transported above, on, or under the water surface. A mixed mode of submarine and surface hydrophilous pollination occurs in the seagrass Zostera marina L. In the surface mode of pollination, pollen rafts or ‘search vehicles’ which superficially resemble snowflakes, form at low tide and are transported on the surface of the sea by winds and water currents. Some of the search vehicles collide with the floating female stigmas, effecting pollination. In the submarine mode of pollination, small pollen masses resembling whisk brooms travel beneath the surface of the water. Although we failed to observe a submarine pollination event, SEM analysis of stigmas from subtidal populations confirms that submarine pollination does occur in Z. marina. However, observations of stigmas positioned at and below the surface of the water show surface pollination to be highly efficient. Electrophoretic evaluation of both subtidal and intertidal populations indicates significant genetic variation between populations. Given the high flux rates of surface-borne pollen and pollen viability in excess of 5 h, it is likely that surface-borne pollen is a major source of gene flow in Zostera marina populations.

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.

A multivariate comparison of the seagrasses Zostera marina and Zostera japonica in monospecific versus mixed populations

The seagrasses Zostera marina and Zostera japonica coexist along a portion of a gradual intertidal gradient at Roberts Bank, British Columbia. Individual comparisons of 11 morphological and 2 population characters for Z. marina did not reveal consistently significant differences between the deeper water monospecific population and the population in mixed vegetation. Similar comparisons between the shallow-water monospecific Z. japonica and the mixed vegetation indicated a suppression of Z. japonica shoot production (density) under a canopy of Z. marina. Principal components analysis was used to summarize the original data into a new set of multivariate characters that were then compared using ANOVA and MANOVA. With these analyses, the subtle differences between Z. marina populations became significant at various times. Similar analyses for Z. japonica indicated that morphological differences between the monospecific and mixed populations persisted throughout the sample period. The multivariate characters provide summaries of populations that are useful in detecting subtle responses to varied environments. Key words: Zostera marina, Zostera japonica, seagrass morphology, multivariate analysis, MANOVA, intertidal zone.

Electrophoretic polymorphism and divergence in Najas marina L. (Najadaceae): Molecular markers for individuals, hybrids, cytodemes, lower taxa, ecodemes and conservation of genetic diversity

Isozyme polymorphism in leaves and seeds has been studied in 23 populations of Najas marina L. subsp. marina from Europe and the Far East, in 20 populations of N. marina L. subsp. intermedia (Gorski) Casper from Europe, the U.S.A. and Turkey and in eight populations of N. marina L. subsp. armata (Lindb. f.) Horn af Rantz. from Israel, Egypt and Burundi. These three population groups are designated as A, B and C, respectively. Alcohol dehydrogenase (ADH), glucose-6-phosphate dehydrogenase (G6PDH), glutamate dehydrogenase (GDH), isocitrate dehydrogenase (IDH), malate dehydrogenase (MDH), malic enzyme (ME), peroxidases (POD), 6-phosphogluconate dehydrogenase (6PGDH), shikimate dehydrogenase (SkDH), superoxide dismutase (SOD), xanthine dehydrogenase (XDH), glucose-phosphate isomerase (GPI) and a total protein fingerprint with sodium dodecyl sulphate (SDS) have been investigated thoroughly. ADH, IDH, ME, POD, SkDH, SOD, XDH, and GPI isozymes are polymorphic and can be used as a diagnostic tool (i.e. molecular marker) in comparing N. marina populations, while G6PDH, GDH, MDH and 6PGDH are constant in all tests. The genetic variability at 24 loci is rather low, while most of the polymorphism is mainly present in the Adh loci. Single populations seem to be very representative of their subspecies for the alleles in moderate or high frequencies. According to these isozyme patterns and the number of populations investigated here, it may be presumed that subsp. marina is very consistent for different localities in the basins of the Rivers Somme, Meuse, Mosel and Rhine (localities 1–16 and 19) but that there is a slight variation in the ADH patterns of some populations of the Alp region (Localities 17, 18 and 20), and in the SkDH pattern of those from the Far East (Localities 21–23). Subsp. intermedia, however (Localities 24–43), differs from subsp. marina, not only in a certain set of isozyme patterns, but also in the greater variation between and within populations. Allelic variants in two gene loci of ADH are detected in seed. Multiple allelic forms in the Adh-1 gene (Locus 1) are observed in plants from distant localities such as Turku, Ober-Uekersee, Kleiner Krienertsee, Gnadensee and Sempachersee (Localities 26, 32, 33, 38–40 and 41, respectively). Multiple allelic forms in the Adh-2 gene (Locus 2) are observed in plants from distant localities such as Turku, Rohr-Pöhle, Grosser Mechowsee, Faulersee, Waschsee, Gnadensee and Sempachersee (Localities 26, 34, 35, 36, 37, 38–40 and 41, respectively). A certain variation among populations is observed in ME isozymes. Subsp. armata (Localities 45–51) also differs from subsp. marina and comes very close to subsp. intermedia. Cluster analysis of electrophoretic data revealed that the exact delimitation between subsp. armata and subsp. intermedia remains difficult. The autotetraploid plants from Merkaz Sappir (Israel) occupy an intermediate position between European subsp. intermedia and the central African subsp. armata. As to the evolution of these three taxa, it is presumed that they are not derived from each other, though Karyotype A could be derived from Karyotype B and vice versa, but that they are derived from a common ancestor. The highly polymorphic subsp. intermedia and subsp. armata most likely would be closer to such a common ancestor than the subsp. marina which exhibits more genetic divergence. Enough variability is observed within the species to provide an attractive model to study and use molecular markers as a genetic basis of character correlations in its infraspecific taxa. With isozymes of N. marina L. s.l., direct information might be obtained on: the karyotype of individuals (using leaves as well as seeds); the presence of accessory chromosomes within a population (using ADH in seeds); the autotetraploid nature of individuals (using ADH in seeds) and hybrids (using ADH and ME in seeds, POD and SkDH in leaves). The investigation on natural populations reveals biogeographical patterns, such as the distribution of different alleles in ADH of the seeds. They are valuable as markers for ecodemes and in particular for the conservation of their genetic diversity.

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.