To achieve a high rate of primary production, an aquatic plant community must achieve a high rate of collection of light energy and an efficient utilization of this absorbed energy by its photosynthetic system, followed by conversion of photosynthate to new cell material. In this section we are concerned with the ways in which some natural and man-made ecosystems manage to do this.
We have discussed earlier (Chapters 3, 9 and 10), the major limitation on aquatic photosynthesis imposed by the fact that the plants have to compete for photons with the other light-absorbing components - water, CDOM, tripton - present in the aquatic medium. The most effective strategy that aquatic plants can follow to minimize the extent to which their photosynthesis is limited by lack of light is to attach themselves to surfaces at depths sufficiently shallow to ensure that for most of the day sufficient light is available to permit active photosynthesis. By 'choosing' to be benthic rather than planktonic, plants avoid the problem of being carried by water circulation down to depths where the light is insufficient for growth. Furthermore, the attached benthic flora can, as a consequence of water movement (tides, currents, wind-induced circulation), exploit the nutrients in a much larger volume than that in its immediate vicinity at any moment. The non-motile members of the phytoplankton, by contrast, are carried around in the moving mass of water and so have less opportunity to find new nutrient supplies. The rooted members of the benthic flora - seagrasses in marine waters and the various kinds of aquatic angiosperms in fresh waters - have the additional advantage that they can derive nutrients from the sediments in which they are growing.
It is thus not surprising that the most productive natural aquatic plant communities are benthic. In marine waters the most productive systems are brown algal beds, seagrass beds and coral reefs. The kelps, brown algae of the genera Laminaria and Macrocystis, form dense forests in the sublittoral zone of rocky coasts in cool waters. According to Mann and Chapman (1975), these plants achieve annual net production rates in the range 1000 to 2000 g Cm"2. The intertidal brown seaweeds such as Fucus and Ascophyllum in temperate and subarctic latitudes have annual net production rates of 500 to 1000 g Cm"2. Tropical sublittoral sea-grass beds dominated by the genus Thalassia achieve annual net production rates of 500 to 1500 g Cm"2 and in temperate waters, beds dominated by genera such as Zostera, fix in the range of 100 to 1500gCm~2. In Posidonia oceanica meadows in the Mediterranean, Pergent et al. (1997) estimate that 32 to 36% of the carbon fixed annually is stored in the sediments in the form of rhizomes and attached leaf sheaths. For the globe as a whole, Duarte and Chiscano (1999) calculate, on the basis of literature data for 30 species, an average seagrass annual production of 1012 g dry weight m~2 yr"1, corresponding to ~340gCm~2yr_1. They estimate that the average surplus carbon produced by seagrasses globally is about 0.16 Gt per year, which corresponds to 15% of the total excess carbon produced in the global ocean (i.e. the net CO2 uptake by oceanic biota).
In coral reefs primary production is carried out by multicellular algae and seagrasses, as well as by the symbiotic zooxanthellae living within the cells of the coral. Total annual primary production on coral reefs is typically in the range of 300 to 5000 g Cm"2.801,968
If the immediate products of photosynthesis, which are carbohydrate in nature, are to be used for cell growth and multiplication, proteins, nucleic acids and other cell constituents must be synthesized, for which mineral nutrients are required. A problem faced by aquatic plants is that at the time of year when high solar altitude favours photosynthesis in any one species, it favours it in all so that the period of maximum potential primary production is also a period of low nutrient levels in the water. Certain productive brown algae in the genus Laminaria circumvent this problem by separating the period of maximum growth from that of maximum photosynthesis.667,831,862 In the summer, the plants accumulate photosynthate in the form of mannitol and laminaran, but grow only slowly. In the winter, although the light intensity is low, the nutrient levels in the water are at their highest and this is when the kelps grow rapidly at the expense of their stored carbohydrate.
Taking the world ocean as a whole, most of the primary production is carried out by phytoplankton. Within coastal embayments and estuaries, however, it is commonly the case that the benthic macrophytes account for most of the primary production. For St Margaret's Bay, Nova Scotia, Canada, Mann (1972) found that seaweed (mainly brown algae) productivity averaged over the whole 138 km2 of the bay was 603gCm~2yr_1, compared to 190gCm~2yr_1 for phytoplankton. In the Newport River estuary, North Carolina, USA, the contribution of seagrasses to annual primary production was estimated to be 2.5 times that of phytoplankton.1350
Beds of submerged macrophytes in inland waters have productivities comparable to those of marine macrophytes. In the temperate zone, values range from below 10 to about 500gCm~2yr_1, whereas in tropical regions annual production may exceed 1000 gCm~2.1455
The major factor responsible for the lower productivity of the phyto-plankton, compared to the benthic plants, is, as we have noted, the lower average irradiance they receive as a consequence of vertical circulation. One solution to this problem, adopted by the blue-green algae (§12.6), is the evolution of a flotation mechanism enabling the algae to move vertically within the water to a depth that suits them, without (as in the dinoflagellates) the need for continual expense of energy to drive flagella. This may in part account for the fact that the natural water bodies with the highest phytoplankton productivity are dominated by blue-green algae: annual net yields from such water bodies are commonly in the region of 300 to 1000 gCm~2 but values in the region of 2000 gCm~2 have been reported.520
The circulation problem is also in some measure solved if the water body is very shallow, so that the cells never get very far away from the light. This is the solution generally adopted in man-made high-yield aquatic ecosystems such as sewage oxidation ponds or algal mass culture systems1342 in which a high rate of phytoplankton primary production is the aim: depths of 10 to 90 cm are commonly used. It is generally arranged in such systems that mineral nutrients are available in excess, so that the rate of primary production is limited by the rate of supply of PAR, and sometimes also of CO2, to the system. Bannister (1974a, b, 1979) has developed a theoretical treatment of phytoplankton growth under mineral nutrient- and CO2-saturated conditions by means of which the steady-state growth rate can be expressed as a function of the incident irradiance and the photosynthetic and respiratory characteristics of the cells. Goldman (1979a, b) has comprehensively reviewed the topic of outdoor mass culture of algae. He concludes that yields in excess of 30 to 40 g dry matter m-2 day(equivalent to -5000 to 7000gCm~2yr_1) are unlikely to be exceeded: to date, such rates have been achieved only for short periods (less than a month), and over the long term, yields of 10 to 20gm-2day-1 are more typical. A crucial limiting factor in such systems is light saturation of photosynthesis. Even in the very dense algal suspensions that develop, the cells near the surface are exposed to irradi-ances well above their saturation points. Of the three requirements for high productivity listed at the beginning of this section, such cells will have a high rate of collection of light energy, and (given the plentiful nutrient supply) rapid conversion of photosynthate to new cell material: what they will lack is efficient utilization of the absorbed energy in photosynthesis, since their carboxylation system will be unable to keep pace with the high rate of arrival of excitation energy. The inability of algae to increase their photosynthetic rate linearly with irradiance all the way up to full sunlight is likely to remain an insuperable obstacle in the path of increasing yields from outdoor mass culture. To develop, by mutation or genetic engineering, algal types with increased carboxylase and lowered pigment content would not be a solution, because such cells would be poorly adapted to the lower light intensities existing at greater depths. It is essential that the algal suspension should be sufficiently deep and dense to absorb nearly all the light incident upon it. This means that a gradient of irradiance, from full sunlight to near darkness, must exist within the suspension, and no one type of alga can be optimally adapted to all the intensities present. In principle, a laminar arrangement with a range of algal types stacked one above the other in thin layers separated by transparent boundaries, with the most Sun-adapted type at the top and shade-adapted type at the bottom, could increase the yields, but the technical problems and capital cost would be considerable.
Krause-Jensen and Sand-Jensen (1998) have sought, on the basis of a wide-ranging literature review, to determine whether general relationships between chlorophyll concentration, light attenuation and gross photosynthesis, across phytoplankton communities, macrophyte stands and microalgal mats can be arrived at. In the sea, phytoplankton chlorophyll a concentrations are typically in the range 0.02 to 0.2 mgm-3
in the oligotrophic regions, 0.5 to 10.0 mgm~3 in productive waters (upwelling regions, eutrophic coastal) and somewhere in between in the mesotrophic regions. In eutrophic inland waters, chlorophyll a concentrations can rise to 100 mgm~3 or more. For macrophyte stands, Krause-Jensen and Sand-Jensen estimate the volumetric chlorophyll a concentrations to be in the range 200 to 15 000mgm~3. Benthic microalgal mats had the highest chlorophyll a concentrations - up to 700000mgm~3. Along with increasing chlorophyll concentration, the photic zone diminishes from >100m in sparse phytoplankton communities, to centimetres/ metres in macrophyte stands, down to <1 mm in dense microalgal mats. The compression of the photic zone is accompanied by a corresponding increase in the rate of photosynthesis per unit volume. Integral photosynthesis (photosynthesis per unit area), however, has an upper limit of ~60 mmol O2m~2hr_1, at midday, this being achieved by dense macrophyte and phytoplankton communities in which most of the incident light is captured by the plants. Benthic microalgal mats do not appear to be able to attain quite such high rates, being apparently unable to exceed ~38mmolO2m~2hr~1. In the dense phytoplankton and macrophyte communities, maximum integral photosynthesis seems already to be achieved when the plants capture as much as 50% of the incident light, no further benefit being obtained by higher proportional light capture, suggesting that other factors such as CO2 exchange limit integral productivity at high plant densities.
A particular type of productive aquatic system that has become more common in recent decades is that referred to as harmful algal blooms (HABs), or sometimes as red tides or brown tides, although not all red or brown tides are harmful. Of the 60 to 80 phytoplankton species that have been reported to be harmful, 90% are flagellates, mainly dinoflagellates.1233 It is the peridinin or fucoxanthin pigments of the algae that impart the characteristic colour to the water. The topic has been reviewed in a special issue, The ecology and oceanography of harmful algal blooms, of Limnology and Oceanography (Vol. 42, No. 5, 1997), and in a symposium on Molecular, Cellular and Ecophysiological Bases of Noxious and Harmful Algal Blooms, of the Phycological Society of America, 1998, papers from which were published in the Journal of Phycology (Vol. 35, No. 6, 1999).
Harmful algal blooms can have a variety of undesirable effects, but the one that causes most concern is the production by some species of toxic metabolites that can kill other marine life forms, including fish, shellfish, mammals and birds, and can also poison human beings.1232,906 These blooms have caused major economic losses to aquaculture - farmed fish and cultivated shellfish - in many coastal sites around the world. Increased anthropogenic nutrient enrichment of coastal waters is thought to play a role in the increased occurrence of HABs. In the case of dinoflagellates, the worst offenders, which as we noted earlier (§11.3) are particularly sensitive to turbulence, bloom development is favoured by stratification. Smayda (1997b) suggests that HAB flagellates have evolved four adaptations that assist their bloom-forming propensities. Their ability to carry out vertical migration enables them to retrieve fixed nitrogen from the nitrate-rich layer below the pycno-cline. Many flagellates have the ability to take up dissolved and/or particulate organic matter from the water. Some species excrete compounds that inhibit the growth of other members of the phytoplank-ton (allelochemical interspecific competition). They excrete icthyotoxins that protect them from grazing by fish larvae and/or toxins that protect them against grazing by copepods (allelopathic defence against predation).
In the case of one particular toxic red tide dinoflagellate, Karenia brevis (formerly known as Gymnodinium breve) in the Gulf of Mexico, the physical and ecological events that led to it becoming dominant have now been studied in great detail.1437,1436 Off the west coast of Florida, red tides of K. brevis, after sporadic occurrences in the first half of the twentieth century, have occurred virtually every year since 1945. In the western Gulf of Mexico, off Texas, K. brevis red tides, formerly rare, are now becoming of annual occurrence.1436 On the basis of a major research project involving a large number of workers and many institutions, Walsh et al. (2006), following earlier work by Walsh and Steidinger (2001), have arrived at a working hypothesis to explain the phenomenon. Wet deposition of iron-rich dust, transported in the wind across the Atlantic Ocean from the Sahara Desert, allows, in these already phosphorus-rich waters, the nitrogen-fixing cyanobacterium, Trichodesmium, to bloom in this otherwise nitrogen-poor sea water, thus providing fixed nitrogen for other phytoplankton. Onshore upwelling of seed populations of K. brevis takes place into coastal surface waters in which, because of their high levels of CDOM, light inhibition of icthyotoxic K. brevis is alleviated, permitting a small red tide to a level of — 1mgchl am~3. Fish mortality ensues, and the dead fish serve as a supplementary nutrient source, giving rise to large red tides at concentrations —10mgchl am~3, sufficient to provide adequate self-shading for this somewhat light-sensitive species. Walsh et al. note that Karenia spp. have, particularly within the last decade, caused toxic red tides in similar coastal habitats around the world, downwind of the Gobi, Simpson, Great Western and Kalahari Deserts, and suggest that this is a global response to both desertification and eutrophication.
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