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The density of the 40 most abundant demersal fish species off Namibia is inversely scaled to their average length, similar to the scaling of mortality to fish length. However, the density of individual fish species is, on average, scaled to the −2 power of average fish length in the aggregates, indicating that biomass increases (slope > −3) during the growth of most species examined. These findings indicate that size structure of these fish populations is primarily determined by mortality.
In the previous chapter we discussed the ways seagrasses obtained carbon, nitrogen and phosphorus from the environment, elements that are vital for their structure and functioning. As tissues die, these elements are again lost from the plants, although resorption processes may somewhat mitigate the loss rates. The plants thus have a direct influence on the dynamics of chemical elements in their environment. Uptake by, and loss of, elements from the living plants are only two aspects of the fluxes of matter in seagrass systems. In this chapter we will focus on the various processes determining these fluxes, with particular attention to those relevant to the dynamics of carbon, nitrogen and phosphorus. A variety of processes, biological, physical and chemical, plays a role in shaping the dynamics of these elements, but they share one feature in common: directly or indirectly they are influenced or even determined by the presence of the key species in the system, the seagrasses. Primary production and mineralization are two major processes driving the carbon and nutrient dynamics within the seagrass system, these processes coinciding with fixation and release of inorganic compounds, respectively. Inorganic nitrogen- and phosphorus-containing compounds released during mineralization can be captured again for the production of plant biomass. Although much of the plant biomass dies without being eaten by herbivores and is directly processed by the decomposer community, some of it is consumed by herbivores.
Data on nutrient contents of 27 seagrass species at 30 locations were compiled from the literature.Mean ( f SE) concentrations of carbon, nitrogen and phosphorus in seagrass leaves were 33.6 2 0.31, 1.92 f 0.05, and 0.23 2 0.011 % dry wt, respectively.The median C:N:P ratio was 474 :24: 1, which represents a C:P ratio more than 4 times, and a N:P ratio more than 1.5 times that of oceanic seston.These ratios are, however, less than those previously reported for marine macrophytes (550 : 30 : 1) by Atkinson & Smith (1984).Nitrogen and phosphorus variability within species was large, but carbon contents exhibited little variability.Accordingly, carbon:nutrient (N and P) ratios were inversely related to changes in nutrient content, and the rate of change in C:N and C:P ratios with increasing nitrogen or phosphorus content in plant tissues should shift from high to small as nutrient supply meets the plant's demands.The median nitrogen and phosphorus contents reported here (1.8 % N and 0.20 % P as % DW) correctly discriminated between seagrass stands that did or did not respond to nutrient enrichment, thus offering a useful reference for comparisons of seagrass nutrient contents.
Both water transparency and lake latitude influence the depths of maximum biomass (Z b ) and maximum depth of colonization (Z c ) of submerged plants. The differences in the depth distribution of plants in lakes differing in water transparency become more pronounced as latitude decreases. Changes in transparency in low-latitude lakes should result in greater changes in macrophyte cover than similar changes in lakes at higher latitudes. The maximum depth of colonization appears to be largely a function of water transparency, whereas the depth of maximum biomass is best related to latitude. Relationships developed here allow better predictions of Z c and Z b for individual lakes than were possible before.
