Biomass turnover

Rates of photosynthesis

Effects of light quality on kelp physiology

Summaries of sporophyte growth for kelp species found in the UK

As a general rule, the kelp species found in UK waters grow at rates much faster than might be assumed from the size of plants present in the field. The sporophyte phases in the life cycle of members of the Laminariales grow in a manner somewhat like a conveyor belt. The meristematic tissue is at the junction between the stipe and the lamina - the further away from the meristematic region the older the cells. At the distal end of the blade, tissue is being lost continually through decay, abrasion against surrounding plants or wave action, and this represents a significant contribution to the POM (particulate organic matter) in coastal waters (Wilkinson, 1995). As a result, the biomass that a kelp plant contributes to the coastal ecosystem over a year can be more than 2 to 3 times the biomass of the plant itself. Large portions of the blade of a kelp plant can be removed and regrowth can occur if the basal area of the blade remains, but, as the energy reserves held in the blade have been lost to the plant, regrowth is slow (Lüning et al., 1973). If the meristematic or transition tissue is removed, the stipe and holdfast degenerate and the plant is lost from the population.

The photosynthetic rate of whole kelp plants varies with the age of the plant and the time of year, but the variations in the rate of photosynthesis among a group of individuals of the same species can be greater than the rate variations of the population due to seasonal changes (Birkett, Dring & Savidge, in prep.). The rate of photosynthesis of different parts of a kelp plant depends on the age of the tissue sampled (i.e. the distance of the sample from the meristem). Net production estimates based on photosynthetic measurements suggest that kelp plants contribute significantly to the DOM (dissolved organic matter) in coastal waters (C. McQuaid, pers.comm.) as well as to the POM and that simple growth measurements in the field or the laboratory may be inadequate estimates of the growth of kelp plants.

The absorption of light in coastal waters is much influenced by the amount of particulate matter in suspension (biotic and abiotic) as well as by the dissolved organic components which increase the absorption of blue wavelengths (see section II.C.). Wavelengths of light are attenuated differentially as a result of these factors, altering the spectrum of wavelengths available at different depths. Blue wavelengths are rapidly attenuated in coastal waters. The proportions of red and blue light within the spectrum available to kelp plants has a significant effect on the rates of photosynthesis, development and growth of the plants. Red light favours the accumulation of carbohydrates and blue light enhances protein synthesis, respiration and enzyme activation (Dring, 1988). The light-saturated rate of photosynthesis in blue light may be 50-100% higher than in red light (Dring, 1989). These effects may have a strong influence on kelp distribution and density within a kelp biotope, particularly where land drainage introduces significant concentrations of light-absorbing particles or humic chemicals into coastal waters.

This is a perennial plant (see table at end) which lives for about 4-5 years in the Irish Sea (T. Holt, pers. comm.) and may live for 7 years in Norway (Baardseth, 1956). Under suitable conditions of strong water flow and low wave action, plants can achieve a blade length in excess of 2 m. (D. Birkett, pers. obs. in the Narrows, Strangford Lough). Maximum growth rates are recorded in April and May (T. Holt, pers. comm.), with plants in the field (Isle of Man) growing by 20 cm a month. Plants in an adjacent rope culture system had average extension rates of 5 cm per day. In June and July the growth rate slows and the continual distal erosion reduces the plant to a holdfast, stipe and a short length of blade, in which state the plants over-winter. In the late autumn and early winter the sporophylls are produced from the stipe at the base of the blade and these persist after sporulation and remain on the plant as thickened, vegetative structures during the summer months, but their total duration is not known. In the second year of growth, plants may become larger than in their first year (T. Holt, pers. comm.). Slow growth continues during the summer months but cannot compensate for the distal tissue loss.

A relatively short lived perennial plant (see table at end). Mature individuals can produce blade lengths of up to 2.5 m under suitable conditions. On the Calvados coast of northern France, stipe lengths were measured for plants of the same age. In shallower water, the average stipe length was 40 cm and, in deeper habitats, 60-70 cm (Perez, 1969). Maximum rates of growth are reported for the spring and early summer months, with minimum growth rates occurring during the autumn. After their third year of growth, the size of the lamina does not increase, although plants at the end of their fourth year of growth may have laminae that are 1.5 m long, and it has been estimated from the growth of the meristematic region that, over the previous two years, the blades had produced an additional total of nearly 3 m of tissue (Gayral & Cosson, 1973).

As a consequence of commercial interests, the biology of this species has been extensively studied over the years throughout its habitat range. Plants are perennial (see table at end) - at least with regard to the holdfast and stipe - and growth follows a distinct seasonal pattern (Kain, 1971b; Lüning, 1971). The period of most rapid blade growth is between December and June, when a completely new blade develops from the meristem. The old blade, delineated by a distinct constriction of the lamina, remains attached to the new, fast growing tissue until it is lost in the spring or early summer. Although the new blade does not continue to increase in size during the summer months, the metabolic activity of the plant remains high (Lüning, 1971). In the first few years of the life of a plant, the blade area and stipe length increase each year until the plant is over 5 years old (Lüning, 1971). The growth rate and length of the stipe may be reduced with depth in the kelp bed. In France and England, stipes are usually about 1 m long but, on open western coasts and in Scotland, stipe lengths in excess of 3 m have been recorded. The length of the lamina depends not only on the time of year but on the local and recent hydrographic conditions. The onset of blade growth during the winter months is an endogenous circannual response mediated by daylength (Lüning, 1990).

Published information is unavailable for this species in UK habitats; growth patterns and reproduction may be similar to L. hyperborea.

These kelps are short lived perennials (see table at end), and the age of the most distal blade tissue rarely exceeds 6-8 months. Maximum growth rates have been measured during the late winter and spring with minimum growth during the late summer and autumn (Parke, 1948; Lüning, 1979). The plants have been shown to reduce their growth rates in response to the onset of shorter days (Lüning, 1988). The overall length of a mature plant in a sheltered locality may not change much during the growth season due to the distal loss of tissue, but extension growth within the blade has been measured as 1.1 cm per day in May with a total length addition of over 2.25 m tissue per year for plants in their second growth season and somewhat less for plants in their third growth season. (Sjøtun, 1993). Many of the larger plants are lost from the population during the late summer and autumn, the annual loss of tagged plants indicating that the normal age of plants did not exceed 4 years (Parke, 1948; Sjøtun, 1995).

The plants of this species are generally described as annuals (see table at end), but would be more accurately described as monocarpic, as the plants which survive to over-winter are those which did not become fertile in their first summer. Young sporophytes appear in the spring and grow rapidly. The blades of individual plants may reach lengths of over 3 m under ideal conditions (D. Birkett, pers. obs.) and the basal "bulb" may reach a diameter of over 30 cm (Norton, 1970). Blade length increases most rapidly in the summer months (6.2 cm per week; Norton, 1970), without allowing for distal abrasion. Growth of the blade ceases at the onset of fertility, and distal decay and abrasion results in the blade becoming progressively smaller; the stipes are finally lost towards the end of autumn. The stipeless bulbs often survive over the winter months, decaying and becoming detached in the spring. The onset of sporophyte growth in the spring appears to be irradiance-dependent rather than temperature-related.

This kelp has only recently arrived in UK waters (first reported on the south coast of England in 1994; R. Fletcher, pers. comm.). It is an annual species (see table at end), although possibly monocarpic. The first young sporophytes appear in the autumn and the plants grow rapidly (in suitable conditions) during the winter. Maximum growth rates are reported in the spring, at which time the plants become fertile. The fronds may reach a length of 1.65 m but start to deteriorate during the summer. By the autumn, any fronds that persist are heavily overgrown by colonial animal species.

Next Section                     References