As outlined in the first chapter, seagrasses are the only angiosperms that are adapted to a marine submerged existence. Basic requirements for growth are similar for terrestrial angiosperms and seagrasses alike. Life in the marine realm, however, implies exposure to environmental conditions that are considerably different in many respects from those in terrestrial habitats, imposing constraints on the availability of some resources, or calling for specific adaptations to acquire others. In this chapter we will focus on environmental resources imperative for growth in seagrasses, i.e. light, inorganic carbon and nutrients, and on the plant properties relevant to their acquisition and use.
Abstract. The dynamics of organic carbon production, release and bacterial use was examined across a range of communities spanning from highly oligotrophic ones in the Subtropical Atlantic Ocean, mesotrophic ones in the Mediterranean Sea and productive ones in the Northern African upwelling and the Southern Ocean. A comparative analysis of experiments examining total and particulate organic carbon production across a range of time scales (15 min to 24 h) for 20 communities with contrasting phytoplankton cell status, as assessed by cell lysis rates, and the use of a simple inverse model was used to resolve patterns of carbon flow in the microbial food web. Communities in productive ocean waters accumulated organic carbon over hourly time scales, whereas only a small fraction of net primary production accumulated in communities from oligotrophic waters. These communities supported high phytoplankton cell lysis rates leading to a rapid flux of organic carbon to bacteria, which had high affinity for phytoplankton-derived carbon, much of which was rapidly respired. Conventional assessments of primary production in the oligotrophic ocean severely underestimate net phytoplankton production, as carbon flow in microbial communities from oligotrophic ocean waters occurs within short (minutes) time scales. This explains difficulties to reconcile estimates of primary production with independent estimates of carbon use by bacteria in oligotrophic marine ecosystems.
The biochemical composition and metabolism of the balloonlike Mediterranean macroalga Codium bursa J. Agardh are strongly size dependent and constrained by the spherical geometry of its thallus. Changes in reproductive phenology, population structure, density, biomass, and production of this widespread alga were followed over an annual cycle to examine whether mortality rate was size and age dependent. Examination of size and age distributions showed the population to be in quasi‐steady state. A main recruitment event occurred in winter, with the highest gametangial densities observed in October, and the peak of recruits observed in December. The small recruits (0–2 cm in diameter) exhibited high growth (1 cm in <1 month) and mortality rates. High exponential mortality rate was also observed in the larger individuals (>12 cm, or 6 years old). Intermediate size C. bursa showed the lowest mortality rate, indicating that organisms within this size range (3–12 cm) escape advective and grazing losses that decimate small recruits and that they, unlike larger organisms, maintain an adequate physiological status. Asexual gemmation within the intermediate size classes may also contribute to maintain stable populations of this species in the NW Mediterranean littoral zone, since recruits derived from sexual reproduction experience extremely high mortality rates.
