Brian Moss

A mechanism to account for macrophyte decline in progressively eutrophicated freshwaters

Abstract Artificial fertilization of lakes ultimately leads to reduction in submerged macrophyte productivity. This has usually been attributed to shading by increasing populations of phytoplankton. Evidence is given that loss of macrophytes is often due to increased growth of, and shading by, epiphytes and filamentous algae associated with the weed beds, and that phytoplankton development is subsequent rather than causative. A new hypothesis is outlined in Fig. 1 and support for it is drawn from palaeolimnological studies, laboratory experiments and field observations and experiments in the Norfolk Broads, an area of shallow, peat-excavated lakes, and from the literature.

The composition and ecology of periphyton communities in freshwaters

The surfaces of aquatic plants are not metabolically inert, yet much evidence seems to suggest that they exert little influence on the composition of the periphytic algal communities associated with them. Other evidence indicates a specificity of the algal communities to particular macrophyte species. Evidence is given here, from a variety of studies, that the influence of host type in determining periphyton community composition is greatest in infertile lakes, but, that in progressively more fertile water, external environmental factors become more important.

The Norfolk Broadland : Experiments in the restoration of a complex wetland

1. The Norfolk Broadland comprises wide river valleys, floored with deep deposits of peat and clay. Over forty mediaeval peat pits (the Broads) became flooded after the fourteenth century and were mostly connected with the rivers by navigation channels. Between about 1400 A.D. and 1800 A.D. the valleys supported a diverse wetland ecosystem, partly maintained by deliberate cropping of wetland plants. Some of the wetland was gravity‐drained, but extensive aquatic habitats held diverse fens and submerged vegetation dominated by short‐growing aquatic plants in very clear water.

Regression of Phragmites australis reedswamps and recent changes of water chemistry in the Norfolk Broadland, England

Since 1945, there has been progressive and extensive loss of reedswamp from the margins of a set of small shallow lakes, the Norfolk Broads. This has coincided with the grazing of birds and coypu, increases in disturbance by boats, and the eutrophication of the water. Reed, Phragmites australis (Cav.) Trin. ex Steud., performance has been measured throughout Broadland and was not reduced in Broads where reedswamp is known to have regressed. Regression was strongly associated with the presence of floating mats of reed; the potential effects on these mats of recent changes in nitrate-nitrogen relative to potassium concentrations in Broads water were investigated in controlled growth experiments. An increase in the ratio of nitrogen to potassium supplied decreased rhizome to shoot biomass ratio and the proportion of sclerenchyma in both shoots and rhizomes. At the highest concentration of nitrate, an increase in the supply of potassium reduced the total nitrogen concentration of rhizomes. A low nitrate to potassium ratio in reed biomass also corresponded with the greatest development of sclerenchymatous tissue in rhiizomes from both floating and sediment-rooted growth forms. It is suggested that floating reedswamps have become more vulnerable to disintegration through these changes. This is likely to be compounded by a large decrease in the biomass of horizontal rhizomes in floating reed growing at high nitrate concentrations compared with reed rooted in the sediment.

The composition and ecology of periphyton communities in freshwaters. II. Inter-relationships between water chemistry, phytoplankton populations and periphyton populations in a shallow lake and associated experimental reservoirs (‘Lund tubes’)

Water chemistry, phytoplankton populations and periphyton populations on Potamogeton pectinatus, have been compared in a shallow, brackish lake, Hickling Broad, and in two enclosed, 20 m diameter butyl rubber reservoirs, placed in it. The seasonal changes of periphyton on Hippuris vulgaris and Myriophyllum spicatum in the lake have also been examined. The lake has been made more eutrophic by agricultural land drainage and roosting gull populations, the tubes to a greater extent by gulls perching on their rims. The water in the tubes became persistently dominated by colonial blue-green algae, a spring diatom pulse was absent, and severe nitrogen limitation was found. Periphyton on P. pectinatus in the tubes was scanty. In the lake, however, though phytoplankton chlorophyll a levels were lower, an alternation of spring diatom and summer colonial blue-green algae was recorded. Nitrogen limitation was less severe in the lake and abundant periphyton crops, mainly of diatoms, developed on P. pectinatus and to a...

Conservation problems in the norfolk broads and rivers of East Anglia, England—Phytoplankton, boats and the causes of turbidity

Abstract The Norfolk Broads and rivers of eastern England (Fig. 1) comprise an area hitherto farmed for the diversity of its wildlife and submerged aquatic plant communities. The latter have progressively disappeared since the early 1950s, until only four sites currently retain more than remnants of the original sub-aquatic macrophyte flora and its associated invertebrate fauna. Increases in turbidity of the water have been associated with the loss of macrophytes, and these increase have been variously attributed to phytoplankton and to disturbance of sediment by the many boats of visiting tourist and residents. Synoptic surveys of turbidity were carried out in the navigable waterways of Broadland in summer and winter 1973, and of phytoplankton in summer 1973. The differential distribution of phytoplankton is discussed in terms of the nutrient loadings on, and flushing coefficients of, the waterway. Highly significant correlations were obtained between phytoplankton numbers and turbidity in the system as a whole and Broads and rivers considered separately. A very weak correlation between boat activity and turbidity was shown to be non-causative. It is concluded that increase in turbidity is a function of increased nutrient loading from human activities in the catchment area and that boat disturbance does not contribute significantly to the sustained turbidity.

Algal and other fossil evidence for major changes in Strumpshaw Broad, Norfolk, England in the last two centuries

Strumpshaw Broad is a medieval man-made lake, excavated about 500 years ago in valley peat deposits. Evidence from diatom fossils, and other remains (filamentous algae, Chrysophycean cysts, sponge spicules, snail shells) indicates a series of changes since the eighteenth century probably related to development of the city of Norwich and sophistications of its sewerage system. Progressive nutrient-enrichment led to replacement around 1912 of low-growing macrophytes (Chara) and sparse plankton populations by tall-growing vascular aquatic macrophytes, with increased epiphytic diatom numbers. A blanketing phase of filamentous algae (probably Vaucheria) was associated with the transition between 1795 and 1903. Around 1950, after a phase of increasing plankton populations and declining epiphyte and macrophyte numbers, the macrophyte-epiphyte complex disappeared altogether, to be replaced by dense plankton populations for a few years. These changes and the progressive decrease in volume of the lake owing to incr...

Adaptations of epipelic and epipsammic freshwater algae

SummaryEpipelic algae live freely on sediment surfaces, epipsammic algae live attached to grains in sandy sediments. Both groups may be buried by wave action and animal disturbance when they may find themselves in dark, deoxygenated layers. Epipelic algae, though tolerant of darkness for many days do not survive anaerobiosis for long and must rely on rapid movement to regain the sediment surface. Since they often are non-motile epipsammic algae cannot always move rapidly upwards. They tolerate both darkness and anaerobiosis, retaining considerable photosynthetic potential for several days in the complete absence of oxygen.

Spatial patterns and population dynamics of plant-associated microcrustacea (Cladocera) in an English shallow lake (Little Mere, Cheshire)

Little Mere (Cheshire) is a small (2.7 ha) and shallow (average depth 0.7 m) fertile lake in Cheshire, UK. Nymphaeids cover almost 40 % of its entire surface during the growing season (April to October) and practically all the rest is covered by a mixed community of submerged plants. The lake was intensively sampled for plant-associated Cladocera and zooplankters from April 1998-April 2000. Samples were collected at five sites across the lake, three of them located within lily beds, the other two over submerged plant beds of mixed composition. Specific sampling techniques were developed for floating lily leaves, petioles, submerged plants and water. Significant horizontal differences were identified for most cladoceran species, both open-water and plant-associated, for chydorid periphyton scrapers and for filter-feeders. Daphnia hyalina (L.) and Ceriodaphnia sp were significantly more abundant in lily beds than in more open water in both growing seasons, suggesting lily beds are an effective refuge against fish predation. Size-structure and egg-ratio data support this contention. Egg-ratio models were examined for Daphnia hyalina and Simocephalus vetulus (O.F. Müller), a plant-associated cladoceran. The fertility of S. vetulus in lily beds was generally high throughout growing seasons. The construction of egg-ratio models for this species was hampered by their generally very patchy distributions.

The potential of artificial refugia for maintaining a community of large-bodied cladocera against fish predation in a shallow eutrophic lake

The Norfolk Broads are a series of shallow lakes which are highly eutrophic and typified by dense populations of phytoplankton and an absence of submerged aquatic plants. The zooplankton community is subject to intense predation pressure by young fish and is dominated by small-bodied organisms which have a low potential for reducing phytoplankton populations through grazing. Various designs and densities of artificial refugia for zooplankton against fish predation were established in Hoveton Great Broad in order to enhance populations of large-bodied Cladocera. Initially some of the refuges contained higher densities and larger individuals ofDaphnia andCeriodaphnia than the surrounding open water. However, towards the end of the first season and throughout the subsequent two years, population densities and size-structure were similar both within and outside the refuges, although there was still evidence of enhanced body-size ofDaphnia within the refuges compared with the open water. The provision of habitat structures designed as refugia from fish predation did not enhance large-bodied cladoceran populations enough to promote this restoration strategy as feasible for eutrophic and shallow lakes.