C. Den Hartog

Eelgrass condition and turbidity in the Dutch Wadden Sea

Abstract Populations of eelgrass ( Zostera marina L.) in the Dutch Wadden Sea have witnessed two major phases of decline this century. The first was the total disappearance of sublittoral beds during the wasting disease epidemic of the early 1930s, and their subsequent failure to recover. The second was the gradual disappearance of littoral eelgrass after the mid-1960s. It is argued here that both the lack of recovery of the sublittoral beds, and the disappearance of the littoral population, may have been effected, to a large extent, by changes in turbidity. The increasing turbidity can be linked with progressively increasing eutrophication as well as deposit extracting and dredging activities.

Suffocation of a littoral Zostera bed by Enteromorpha radiata

A large (about 10 ha), mixed bed of the seagrasses Zostera marina L. (narrow morph) and Zostera noltii Hornem. on the intertidal flats of Langstone Harbour, Hayling Island (Hampshire, UK), has been monitored annually since 1986. No noticeable changes took place in the period 1986–1990. In September 1991 this seagrass bed appeared to be largely destroyed by a thick blanket of the chlorophyte Enteromorpha radiata J. Agardh; most still living Zostera plants were in a bad condition. In August 1992 not a single specimen of Zostera was found growing in the area.

Taxonomy and Biogeography of Seagrasses

Seagrasses are aquatic angiosperms, which are confined to the marine environment. The term seagrass (with several linguistic variants in the Germanic language group) refers undoubtedly to the grasslike habit of most of its representatives. The term has been long used by fisherman, hunters, farmers, and other inhabitants of the coastal areas of several European countries, i.e. areas where only species occur with long linear leaves. Ascherson (1871) probably was the first researcher to introduce the term into the scientific literature. The seagrasses form an ecological group, and not a taxonomic group. This implies that the various seagrass families do not necessarily have to be closely related. The taxa regarded as seagrasses belong to a very limited number of plant families, all classified within the superorder Alismatiflorae (Monocotyledonae) (Dahlgren et al., 1985), also generally known as the Helobiae (Tomlinson, 1982). The subclass Alismatanae (Kubitzki, 1998) is with respect to its contents identical with Alismatiflorae. Three out of four families consist exclusively of seagrasses, viz. the Zosteraceae, the Cymodoceaceae, and the Posidoniaceae. In the past these families generally have been classified as subfamilies of the Potamogetonaceae (Ascherson and Graebner, 1907; den Hartog, 1970). Further studies have shown that the latter family appeared to be too heterogeneous (Tomlinson, 1982; Dahlgren et al., 1985), and had to

“Wasting disease” and other dynamic phenomena in Zostera beds☆

Abstract 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.

Chapter 2 – Seagrass taxonomy and identification key

This chapter describes how the seagrass species can be identified. Seagrasses are defined as flowering plants growing in shallow marine environment. They form an ecological group and not a taxonomical one. . They are represented by twelve genera, which have been classified in diverse taxonomic groups. These are the entire families: Zosteraceae (three genera), Posidoniaceae (one genus), Cymodoceaceae (five genera), and three of the seventeen genera in the family Hydrocharitaceae. Seagrasses are aquatic plants, which have fewer morphological and anatomical features for species identification than terrestrial flowering plants. Some species have a vast geographic distribution and occupy different niches; this may result in considerable morphological variation, and some of these variations, when studied more closely, may turn out to be separate taxa. All chromosomal studies on seagrasses indicate that chromosome numbers may be different between genera and families, but that the basic numbers are similar among the different species within the same genus. Classification of the various seagrasses in the taxonomic system, and even more their phylogeny, will continue to be debated for many years to come. The definition of seagrasses at the species level is far from satisfactory in certain genera, and can cause difficulties in arriving at a correct identification. Thus, researchers should be careful in citing and interpreting the published literature, which may contain incorrect species names.

Comparison of a current eelgrass disease to the wasting disease in the 1930s

Abstract A comparison is made of the wasting disease that struck the whole Atlantic population of Zostera marina L. in the 1930s and a current outbreak of a rather similar disease in Z. marina beds along the north-eastern coasts of the U.S.A. Although the disease phenomena on the plants appear to be very similar, disease-related declines of Z. marina are at present still very local. In Europe, diseased plants have been found, but no declines have been observed. The wasting disease in the 1930s was not investigated before the epidemic reached a devastating stage. Present observations may indicate that a new widespread die-off may be developing. In order to facilitate the study of the current epidemic, a scenario of disease and related decline, with several variants, has been elaborated, based on the existing knowledge of the epidemic of the 1930s, but also clearly showing the gaps in this knowledge.

Changes in the seagrass populations of the Dutch Waddenzee

Abstract 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.

Ammonium and nitrate uptake by aquatic plants from poorly buffered and acidified waters

In the Netherlands, atmospheric deposition of ammonia compounds, particularly ammonium sulphate, is an important source for the acidification of oligotrophic soft waters. As a consequence, the acidified waters are generally nitrogen enriched, ammonium being the dominant N form. In this study, it is examined how this alteration in the nitrogen household affects the aquatic plant communities in acidifying waters. The uptake of ammonium and nitrate by leaves and roots of two groups of freshwater plants has been studied using glass incubation chambers. The forst group (Littorella uniflora (L.) Aschers.; Lobelia dortmanna L.; Luronium natans (L.) Raf.; Echinodorus ranunculoides (L.) Engelm.) is characteristic of nitrogen-poor soft waters, whereas the second group (Juncus bulbosus L.; Sphagnum flexuosum Dozy & Molk.;Agrostis canina L.; Drepanocladus fluitans (Hedw.) Warnst.) often occurs in dense stands in nitrogen-enriched, acid waters. Both groups have typical adaptations to the nitrogen condition of their aquatic environment. The soft-water species show a nitrate-dominated (63–73%) nitrogen utilization, with the roots as the major (83%) uptake site. Moreover, they are able to survive at very low nitrogen concentrations. The acid-tolerant species have an ammonium-dominated (85–90%) nitrogen utilization, with the leaves as the major (71–82%) uptake site. This group profits from the increased ammonium levels in acid waters. It is concluded that in the case of acidification increased ammonium concentrations additionally account for the suppression of typical soft-water communities by communities dominated by Juncus bulbosus and Sphagnum spp.

Sabaki River sediment load and coral stress : correlation between sediments and condition of the Malindi-watamu reefs in Kenya (Indian Ocean)

Sediment discharges from rivers have a negative impact on coral reef ecosystems. Indicators of coral decline measured in the present study were: (1) injury to living stony corals; (2) soft coral cover; and (3) bare rocky substrate suitable for colonization by corals. The relationship between these indicators and the distribution of terrigenous sediment was studied for the Malindi-Watamu fringing reef complex along the Kenyan coast off East Africa during 1982 and 1983. Decline of this reef had been repeatedly noted during the preceding decade. The influence of terrigenous sediment from the Sabaki River appears to be strongest in the Watamu area in the south and in the northern-most part of the Malindi reef area. Correlations, between each of the above three coral stress response indicators, on the one hand, and quantitative indicators of sediment loading, on the other hand, were not clear. However, a combined coral stress indicator involving all three factors was shown to have a clear relationship with terrigenous sediment loading and provided a rapid means of field evaluation of the effects of sediment stress on stony corals. Values for the combined coral stress indicator were found to increase in proportion to increasing values of terrigenous sediment loads in both study areas. A higher coral stress indicator value means a high proportion of injured or algae infested corals, and/or a high soft coral cover, and/or a high proportion of rocky substrate suitable for, but unoccupied by, living corals.

Effects of water acidification on the distribution pattern and the reproductive success of amphibians

Nine amphibian species were encountered in poorly buffered waters of The Netherlands (alkalinity ≦1 meq·l−1). These soft water systems are highly sensitive to acidifying precipitation. The number of species as well as the percentage of waters which harbour amphibian populations are strongly reduced in the extremely acid pH-class