Depth distribution of benthic flora

The higher productivity of shallow water applies to the benthic flora also. Any surface within the euphotic zone of a water body is usually found to support a productive plant community. This applies not only to the more obvious macrophyte communities such as kelp forests, seagrass beds, brown and red algal associations on underwater rocks, but also to seemingly bare sand and mud surfaces, which usually harbour a dense microflora among the grains. This microflora, or microphytobenthos, is likely to consist of specialized benthic diatoms in temperate mudflats and sand, or of symbiotic algae within Foraminifera in the sands of tropical lagoons.1272 Moving in a seawards direction, as depth increases so the standing crop of biomass (macrophytes and microflora) per unit area, and the rate of primary production per unit area, both decrease until eventually a depth is reached beyond which the light intensity is too low to support any plant growth. For the important group of large sublittoral brown algae, loosely referred to as the kelps, the lower limit occurs at depths where the downward irradiance is 0.7 to 1.4% of that penetrating the surface: this depth limit ranges from as little as 8 m in the turbid water around the island of Helgoland (North Sea) to about 95 m in the very clear waters of the central Mediterranean.833 For the coralline crustose red algae - commonly the most deeply occurring algal type - the lower depth limit occurs where the irradiance is 0.01 to 0.1% of the subsurface value: this corresponds to about 15 m depth off Helgoland but to as much as 175 m in the Caribbean.833 In the ultra-clear waters east of the Bahamas, Littler et al. (1986) using a submersible, observed a zone of crustose coralline red algae between 189 and 268 m depth, on a seamount. Luning and Dring (1979) found for the sublittoral region off Helgoland that the total downwelling light (400-700 nm) received per year was about 15 MJ m~2 or 70 mol photons m~2 at the lower kelp limit (8 m) and 1 MJ m~2 or 6 mol photons m~2 at the lower red-algal limit (15 m): these may be regarded as the approximate minimum yearly requirements of PAR for growth of these algal types. Much higher annual light requirements (mol photons m~2 yr_1) have been estimated for some freshwater macrophytes by Sand-Jensen and Madsen (1991): 40 to 200 for charophytes (Nitella and Chara), 416 for a bryophyte (Fontinalis antipyretica), 455 for an isoetid angiosperm (Isoetes lacustris) and 1760 for the shallow-water angiosperm, Littorella uniflora.

The depth distribution of aquatic macrophytes is controlled by a number of environmental factors, but the underwater light climate is frequently the primary one.1273 An important ecological parameter for any water body is zcol, the maximum depth to which it is colonized by macrophytes, and this tends to be approximately proportional to the reciprocal of the vertical attenuation coefficient for PAR in the water body, although the constant of proportionality appears to be a function of latitude.1404 For nine New Zealand (North Island) lakes at - 38° S, Vant et al. (1986) found macrophyte depth limits to vary with the attenuating character of the water in accordance with zcol = 4.34/Kd, where Kd was the vertical attenuation coefficient of the lake for PAR averaged over a year. For 18 lakes in the South Island of New Zealand (41-46° S), Schwarz et al. (1996) found that zcol was significantly (r2 = 0.92) related to Kd(PAR) by the relationship zcol = (4.5/Kd[PAR]) - 2.2. Characean algae dominated the deepest communities in 16 of the lakes. For one such species, Chara corallina, from measurements of its photosynthetic properties, of the incident solar radiation over the course of a year, and of the values of Kd(PAR), Schwarz et al. calculated for 17 of the lakes, and for each day of the year, the depth (compensation depth) at which net photosynthesis was equal to zero. This was then averaged for the whole year. They found a linear 1:1 relationship (r2 = 0.86) between the predicted compensation depth and the observed maximum depth of colonization by this alga. For lakes in Scotland and the English Lake District (54-57° N), Spence (1976) found the approximate relationship zcol = 1.7/Kd(PAR). In the case of Vallisneria americana, a common macrophyte in Australian inland waters, Blanch et al. (1998) found the maximal depth of colonization to be predicted by zcol = (8.89/ Kd[PAR]) - 0.06.

For seagrasses Duarte (1991), from a survey of literature data, arrived at the relationship zcol = 1.86/Kd. In the case of seagrass beds, the depth at the lower edge of the bed is sometimes referred to as the ecological compensation depth (ECD), because it is the depth below which the carbon balance for the entire canopy becomes negative and hence unsustain-able.429 For seagrass beds in the Indian River Lagoon, Florida, Gallegos and Kenworthy (1996) found that the depth distribution could satisfactorily be predicted on the basis that the ECD corresponded to -20% light penetration. Seagrass growth was inhibited in the vicinity of the outflow of a canal discharging highly coloured water. Estuaries which are significant waterways are routinely dredged to maintain the navigation channels, an activity which, unfortunately, leads to substantial, and frequently persistent, increases in turbidity. Onuf (1994) carried out underwater light measurements in the Laguna Madre, Texas, over a period covering 3 months before to 15 months after a dredging project in 1988, and concluded on the basis of his data that maintenance dredging had been responsible for the loss of seagrass cover that had taken place in this waterway between 1965 and 1974.

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