Chapter 4 - Marine Plants
Chapter Summary
Anthophytes are flowering plants with a dominant sporophyte generation and a reduced
gametophyte generation. Unlike on land, anthophytes are not the dominant macroscopic plants
in the ocean, but they do have marine representatives that are primarily found near the coast,
such as seagrasses, marsh grasses, and mangals. All types of marine flowering plants host a
unique community of organisms within the habitat they create.
Multicellular algae, otherwise termed “seaweeds,” dominate in the marine environment,
especially brown and red algae. There are three major divisions of algae: the green Chlorophyta,
the brown Phaeophyta, and the red Rhodophyta. Other than morphological features, accessory
pigments are a characteristic used to categorize the algae. Accessory pigments are functionally
and physiologically very important. These pigments capture light of a variety of wavelengths
other than those captured by the primary photosynthetic pigment, chlorophyll a. With the
evolution of accessory pigments, algae have been able to occupy habitats of various depths that
otherwise would have been uninhabitable.
Most algae have common morphological features that can be readily identified. The holdfast is a
structure that anchors the plant to the substrate, the stipe is the "trunk" of the plant that enables it
to grow towards the surface, and the blades contain the photosynthetic tissues and are generally
oriented towards the lighted areas of the region. Other structures, such as gas-filled
pneumatocysts, are present on some species. These structures act as floats that bring the blades
to lighted surface waters.
Reproduction in multicellular algae differs from that of flowering plants. As is common with all
plants, algae undergo alternation of a sporophyte and gametophyte generation and, in general, the
gametophyte generation is dominant with a reduced sporophyte. There are, however, many
variations in terms of alternation of generations for algae and each species needs to be
considered individually.
As introduced in chapter 3, nutrients, especially nitrogen and phosphorus, and sunlight are
necessary for growth and reproduction of phytoplankton. Spatial and temporal variations in
available sunlight, nutrients, and grazers cause significant seasonal and global differences in
marine production. Thus, in many regions of the ocean, these necessities can be limited to a
point where they prohibit population growth. Nutrient amounts and distribution are determined
by physical phenomena. In upwelling regions, nutrient-poor surface waters are continually
replaced by deeper nutrient-rich waters and large ecosystems are supported, such as the
anchoveta ecosystem off the Peruvian coast. In tropical regions, upwelled nutrients are blocked
by a persistent thermocline and surface waters exhibit low rates of production and year-round
plankton communities that resemble those of temperate regions during summer months.
Tropical coral-reef ecosystems rely on the symbiotic relationship between corals and
zooxanthellae rather than conventional phytoplankton photosynthesis (see chapter 9 for details).
We have to consider influences of local phenomena as well as global phenomena to get a sense
of global primary productivity because primary productivity can vary locally. In general, spatial
variations in primary production are common, with mid- and high-latitude regions, shallow
coastal areas, and upwelling zones being the most prolific, but only during warm summer months
when sufficient sunlight is available.