The microphytobenthos

All benthic sediments - whether in tidal mudflats, surf beaches, coral reef lagoons, shallow coastal waters - which receive significant amounts of light, contain communities of microalgae. This benthic community, in any given aquatic ecosystem, is referred to as the microphytobenthos, or sometimes, epipelon. A full account of the epipelon is given in Round's (1981) book, The Ecology of Algae. The microphytobenthos usually consists predominantly of diatoms, but cyanobacteria, dinoflagellates and euglenophytes can also be prominent components.

Planktonic diatoms are unable to move within the water, although they of course undergo passive movement determined by sinking and turbulence. On surfaces, however, diatoms can move by a mechanism involving secretion of mucilage. Diatoms in sediments, both in fresh and marine waters start to move up to the surface of the sediments before dawn.196,1152 In the afternoon the cells begin to migrate down into the sediments again, this movement continuing during the first few hours of darkness, In the case of benthic diatoms in a freshwater lake it was found that they would not leave the surface of the sediments for as long as these were illuminated at 750 lux, but migrated downwards again at light intensities of 75 lux or lower.532 The advantage of moving up onto the illuminated surface during daylight is clear enough: what is less obvious is the benefit the cells derive from burrowing down into the sediments during the hours of darkness - the greater availability of mineral nutrients is a possible explanation.

In turbid estuarine waters, the arrival of the flood tide causes a marked reduction in light intensity. Diatoms in tidal mudflats where the water is turbid can show a tidal rhythm superimposed on the diurnal one. In the River Avon estuary, England, 1 to 2 h before tidal flooding of the sediment at any particular point, the diatoms burrow beneath the surface again: the disappearance of the green or brown film of algae on the surface of the mud moves as a wave along the banks in advance of the tide.1153 In contrast, in the clear water of the River Eden estuary, Scotland, the movement of the benthic diatoms was found to be strictly diurnal.1047 In a tropical intertidal mudflat (Goa, India) Mitbavkar and Anil (2004) found the vertical movement of benthic diatoms to be controlled by both light and the tidal cycle, with light being the overriding influence, but continuous darkness under laboratory conditions induced a tidal rhythm. Diurnal movements onto the surface during the day and down into the sediments during darkness occur with benthic euglena and other flagellates, and blue-green algal species, as well as diatoms. Like other plants, benthic microalgae are susceptible to photoinhibition at high light intensities. They can avoid this by downward migration and a midday diminution of cell numbers at the sediment surface has been found both with euglenoids and diatoms.1153,692

Light penetrates better into sandy sediments than into mud. In intertidal flats in the submerged Wadden Sea (Germany), Billerbeck et al. (2007) found that the depth-integrated scalar irradiance (a measure of the total light available for photosynthesis within the sediment) was 2.04 and 3.45 times as high in fine and coarse sand, respectively, as in mud.

The gross photosynthesis per unit area was about four times as high in the fine sand, and ten times as high in the coarse sand, as in the mud, even though the chlorophyll concentration per unit area was somewhat higher in the mud. Paterson et al. (1998) found that PAR consistently declined to <30% of the surface value within 1000 mm of the surface in intertidal mudflats in the Humber estuary (England), and the euphotic zone (E0[PAR] down to 1%) was limited to the upper 1800 mm. Some areas of sediment in this estuary were dark brown in colour, and were dominated by diatoms; others were light or dark green, and were dominated by euglenids. In intertidal sediments in the Tagus estuary (Portugal) in July, Cartaxana et al. (2006) found that at muddy sites most of the chlorophyll was found in the top 500 mm, whereas in sandy locations relatively constant concentrations were found throughout the sediment profile, down to at least 3.25 mm. Diatoms were the dominant microalgae, but cyano-phytes and euglenophytes were also present. Glud et al. (1999) studied a 5 cm thick microbial mat from a hypersaline salt marsh in Egypt. At its surface, a thin layer of diatoms covered a dense 2 mm thick layer of cyanobacteria (Microcoleus sp.). In terms of oxygen metabolism, three zones were recognized: the upper, diatom zone with a moderate net O2 production; the cyanobacterial zone with a high net O2 production; and a lower zone with disintegrating microalgae and cyanobacteria, and a high O2 consumption rate.

In estuaries and shallow coastal waters, the microphytobenthos can contribute a substantial proportion of total primary production (reviewed in Cahoon, 1999). Photosynthesis is limited by the extent to which light penetrates to the sediment surface, and this is in turn limited by the depth and the optical properties of the water column. For coastal waters of the northern Adriatic Sea, Blackford (2002) calculated, using numerical modelling, that the microphytobenthos contributed more than 50% of primary production in shallow waters (<5m depth), but net primary production decreased to zero at 25 m depth. In the eutrophic Gulf of Fos, NE Mediterranean Sea, Barranguet et al. (1996) found that although the microphytobenthic biomass per unit area, 27 to 379 mg chl a m~2, exceeded the phytoplanktonic biomass, photosynthetic oxygen production by the microphytobenthos, consisting mainly of pinnate diatoms, exceeded that of the phytoplankton only in waters less than 1 m deep. In shallow (5-15 m) coastal waters of the western Seto Inland Sea, Japan, Yamaguchi et al. (2007) found that the increased light attenuation coefficient associated with increased concentration of phytoplankton caused a decrease in the light flux reaching the sea floor, and that the microphytobenthic biomass (1.9-46.5 mgchl am~2) within the upper 10 mm of the sediment was inversely correlated with the phytoplanktonic biomass (10.9-65.0mgchl am~2) in the overlying water column. They concluded that interception of light by phytoplankton was the main cause of the variation in microphytobenthic biomass. At 17 m depth in the Gulf of Trieste (northern Adriatic Sea), Cibic et al. (2008) observed that microphytobenthic primary production was low or negative in the September to December period, and inferred that during these months, associated with low PAR, there was a shift by the benthic microalgae from autotrophic to heterotrophic metabolism. For the shallow (50% <5m depth) Bay of Brest (France, West coast), Ni Longphuirt et al. (2007) estimated average annual production by the microphytobenthos to represent 12 to 20% of total primary production for the ecosystem. In the Ems estuary (Netherlands/Federal Republic of Germany) the microphytobenthos of the tidal flats contributes ^25% to the total annual primary production.294

It is frequently the case in shallow water coral reefs that large areas are occupied by unconsolidated calcareous sediments. The microphyto-benthos at Heron Reef (Great Barrier Reef, Australia) was found by Werner et al. (2008) to be dominated by diatoms, dinoflagellates and cyanobacteria, and in these permeable, coarse-grained sediments, algal pigments were detectable down to a depth of about 8 cm. Net photosynthesis in the sediments was detectable down to depths of 0.5 to 2.0 cm, depending on the site, and was strongly correlated with chlorophyll a content, which varied from 31 to 84mgm~2. The authors estimated that the microphytobenthic production for the entire reef was of the same order of magnitude as that of coral.

The algal cells of the microphytobenthos can be suspended into the overlying water column by tidal movement of,57 or wind-induced turbulence in,294 the water. In the Ems estuary (Netherlands/Federal Republic of Germany) De Jonge and Van Beusekom (1992) estimated that suspended microphytobenthos (mainly diatoms) made up ^22% of the total phytoplankton in the lower reaches, and ^60% in the upper reaches. For the whole estuary they estimated that over 30% of the total chlorophyll a in the water column originated as suspended microphytobenthos. In shallow coastal water over mudflats in the Ariake Sea, Japan, Koh et al. (2007) found that there was a substantial entrainment of sediment microalgae to the water column during the flood tide, and that as much as ^66% of the chlorophyll a in the water column could be derived from the microphytobenthos.

As is the case with the phytoplankton, a major control of microphyto-benthic production is grazing by animals. Gastropods, bivalves and poly-chaete worms, together with the smaller meiofauna, and also some fish (mullet), consume the microalgae of the sediments. When they are suspended into the water column they become an important food source for calanoid copepods.

An underwater microalgal community with some similarities to the microphytobenthos is the community of epiphytes - typically dominated by diatoms, but with filamentous green algae becoming more prominent in eutrophic waters - growing on the surface of the leaves of seagrasses in estuaries, or of freshwater angiosperms in lakes. Where cover is thick, epiphyte productivity can be as much as 60% of the above-ground productivity of the seagrass itself.565 A limiting factor on epiphytic production is grazing by gastropod molluscs, and crustaceans such as amphipods, which consume the epiphyte layer but not the leaf tissue underneath.

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  • Giuliana
    Why iS microphYtoBenthos needed?
    29 days ago

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