NOTE: All of the remaining Divisions have typical higher plant pigments: chl a, b; beta-carotene, several xanthophylls (e.g. lutein, neoxanthin, violaxanthin, zeaxanthin)

DIVISIONS HEPATOPHYTA & ANTHOCEROTOPHYTA

hornworts (Anthocerotophyta) predominantly terrestrial; Anthoceros rarely in permanent lakes

liverworts (Hepatophyta) also mainly terrestrial, but many spp. have aquatic morphs (different from terrestrial forms), esp. in lotic, less commonly in lacustrine waters:

Riccia fairly common along the edge, Ricciocarpus is pleustonic (floating), much like Lemna; both favor eutrophic water Hutch. III, Figs. 22, 23

DIVISION BRYOPHYTA (Mosses)

Hutchinson II, Fig. 4; III, Fig. 24

Order Sphagnales

common in low-Ca, acidic damps soils to lake margins

some spp. submergent, others floating mats: quaking bogs = terminal phase of acid lake history

lake margin Sphagnum communities often have complex spp. zonations related to moisture content, pH, light (shading by trees/shrubs)

Order Bryales

favor soft & moderately hard waters: cannot use HCO₃^- in photosynthesis

most common in lotic, but also present in lacustrine habitats, up to 120 m in Lake Tahoe

like liverworts & Charophytes, can grow much deeper (lower light) than angiosperms, and compete well w/ latter only when substrate is rocky therefore unfavorable for angiosperms

growth at great depth possible only in clear, oligotrophic lakes, so mosses decrease with eutrophication

common, largely aquatic genera incl. Fissidens, Fontinalis, Drepanocladus

TRACHEOPHYTES (Vascular Plants)

subsequent to explosive colonization of land by tracheophytes, a few (1-2%) secondarily adapted to FW (and even fewer into the sea)

many retain relics of terrestrial existence, e.g. a thin cuticle, vestigial stomata, poorly lignified xylem, nonmotile sperm, aerial pollination (most), the latter often insect assisted

aquatic tracheophytes a phylogenetically complex, polyphyletic assemblage

most rooted in substratum, some free-floating in or on water, a few epiphytic

morphological reduction common, both vegetative & reproductive

relevance to humans stems largely from perceived "nuisance" status, interfering w/ navigation, fisheries, irrigation, etc.

DIVISION MICROPHYLLOPHYTA (= subdivision Lycopsida)

vascularized leaves & roots; sporophylls w/ adaxial sporangia

1. Lycopodium ("club mosses", though not really a moss (Bryophyta))

lacks ligules, small membranous appendages at base of "leaves"

most are terrestrial, but L. inundatum occurs in marginal or eulittoral flora in the Arctic

2. Isoetes spp. ("quillworts") Hutch. III, Fig. 26

cosmopolitan, esp. in unproductive softwater lakes

rosettelike w/ simple, unbranched leaves from an underground "corm"

readily outcompeted by angiosperms

primitive vascular anatomy

DIVISION ARTHROPHYTA ("jointed plants")

sole extant genus is Equisetum (= horsetail rushes) Bold et al. Fig. 15-1

common emergent plant in north temperate lakes (up to 1.5 m deep)

whorled leaves & branches which arise alternately from stem nodes (very unusual)

stem is the dominant organ, w/ leaves greatly reduced & essentially vestigial

DIVISION PTERIDOPHYTA (Ferns)

Ceratopteris - pan(sub)tropical genus, pleustonic at or near surface Hutch. II, Fig. 4H

Order Marsilales/ Family Marsileaceae (water clovers) - incl. 3 widespread (esp. warmer areas) genera, 2 occurring in Oklahoma: Marsilea ("water clover"; quadrifoliate leaves) & Pilularia ("pillwort"; filiform leaves) Hutch. II, Fig. 4G

perennial herbs rooting in shallow water or moist soil

erect, spreading or floating leaves

sexual reprod. by sporocarps borne from rhizome; contain both micro- & megaspores

Order Salviniales/ Family Salviniaceae (water ferns) - only 2 extant genera, widespread esp. in warmer climates: Azolla & Salvinia (most spp. native to Africa, only 1 in U.S.), only the former known from OK Hutch. II, Fig. 4E,F

small floating plants, plants monoecious but us. w/ dioecious sporocarps

Azolla - branching stems, alternate bilobed leaves, true roots, and hosts symbiotic Anabaena azollae (Cyanophyta) which probably fixes N₂

Salvinia - simple (unlobed) leaves, lacks true roots (modified filiform leaf segments)

DIVISION CONIFEROPHYTA (Gymnosperms)

Gymnosperms in general are notably absent from aquatic habitats. In southeastern U.S., Taxodium distichum (cypress) is a common swamp/wetland tree. Hutchinson considers it of marginal limnological importance.

DIVISION ANTHOPHYTA (Angiosperms or flowering plants)

Difficult to define aquatic angiosperms b/c many or most terrestrial plants may at some time become submersed.

Sculthorpe lists 28 families (excluding Pteridophytes) of "more or less exclusive hydrophytes", 4 of which are all or predominantly marine/brackish.

Of the 24 FW families sensu Sculthorpe, 14 are listed by Nelson & Couch and 3 more by Tyrl as having OK reps; most of the remaining 7 families are (sub)tropical. However, Nelson & Couch list 9 other families not included by Sculthorpe. Obviously, it is a subjective call, esp. on local/regional scale where only one or a few oddball spp. from a family may occur.

Sculthorpe Table 1.2a, b; Hutch. III, Table 6; Raven et al. Table 16-1

In general, hydrophytes believed to be evolutionarily recent descendents of terrestrial ancestors*, and as a rule favor high light (shallow water). Phylogeny seems uncertain. *An exception may be Ceratophyllum, which DNA evidence suggests to be one of the most basal clades of angiosperms (see Raven et al. 1999, p. 526).

Whereas in terrestrial plants dicots outnumber monocots by 4:1 or 5:1, FW monocots outnumber dicots, esp. in lentic/lacustrine systems.

Among dicots the Ranales (prob. most nearly albeit distantly related to monocots) are the best represented order, while the largest dicot family, Asteraceae, has few aquatic spp., as do the woody families.

Hutchinson (Vol. III, p. 80) summarizes as follows:

"A number of otherwise terrestrial families may contain isolated aquatic genera and species, but most of the characteristic lacustrine plants, particularly those that are submersed, belong to a restricted number of specialized families containing few genera of immensely wide distribution.... Within such widely distributed genera, there are sometimes, as in Potamogeton, very many spp., but more usually the number of species is quite small.... The range of the average species of water plant is greater than that of the average land plant*.... Within a genus, large or small, species of immense range, e.g. P. crispus and Ceratophyllum demersum, may occur... essentially throughout the world."

* I presume this reflects the susceptibility of terrestrial but not aquatic plants to water availability and temperature extremes.

Life Forms/Ecological Classification of Tracheophytes

Ecological perspective: life/growth form of aquatic plants prob. more useful than phylogeny.

Bases for categories incl. morphological similarities (convergence) and/or habitat characteristics, namely relation to water level and substratum.

Sculthorpe Table 1.1; Hutch. III, Table 7; Riemer Fig. 6.1

Hutchinson notes the following generalizations (no doubt many exceptions; refer to Table 7 for codes):

larger, oligotrophic, exposed lakes	graminid emergents (B.I.a) nearshore floating-leaved plants (B.II.a,b) sheltered "offshore" * submerged isoetid rosulates (B.III.b.3)
smaller, eutrophic, sheltered lakes	more foliaceous emergents (B.I.f) nearshore more prominent floating-leaved flora * submerged plants dominated by vittate forms (B.III.a)

* This distinction may suggest differential relative reliance on sediments vs. water column for nutrition.

Salviniids (A.I.b) and eichhornids (A.I.d; e.g. water hyacinth) are often transitory pests.

Smaller pleustonic spp. (A.I.a) are mainly in the littoral of eutrophic lakes.

Other pleustonic plants (A.I.c,e) occur lakeward of floating-leaved plants.

Vascular Plant Biogeography (Sculthorpe)

~30-35% hydrophytes = cosmopolitan/intercontinental (esp. north temperate zone w/ large contiguous land mass); difficult to explain given wide separation of populations by land masses and oceans: spread by migratory waterfowl?

~40% hydrophytes = continental/regional; this is a much lower % than for terrestrial plants

~25-30% hydrophytes = endemic (very restricted) spp.; mostly tropical forms

Hydrophyte (or any organism) biogeography should not be thought of as static: ranges are constantly and presently expanding and contracting.

EMERGENT FOLIAGE: STRUCTURE & PHYSIOLOGY

Remember that hydrophytes evolved from terrestrial ancestors, NOT directly from more primitive aquatic forms, and most retain some traits that are maladaptive to aquatic existence

No major differences in anatomy of emergent structures of hydrophytes vs. terrestrial plants

Young, initially submerged foliage must often tolerate low O₂, and usually form arenchyma tissue near the water surface

Subtle changes in still submerged developing plants include:

decreased leaf thickness & hairiness, stomatal density and cuticle thickness

leaf shape alterations

increased spongy mesophyll volume

Emergent Leaves

1. Monocots (e.g. Typha, Phragmites)

rhizomatous w/ more or less linear leaves

stomata on all surfaces of leaves

typically no palisade/spongy mesophyll distinction

colorless epidermal cells (except guard cells)

generally typical monocot vascular bundle anatomy & cell types

2. Dicots (e.g. Ludwigia)

most produce erect leafy stems, w/ leaves having typical terrestrial dicot anatomy

often stomata on both surfaces

colorless epidermal cells (except guard cells)

distinct upper palisade/lower spongy mesophyll

petiole w/ ring of widely spaced vascular bundles in transsection and lacunate cortex

3. Physiology

main problem is hypoxia below water, esp. near sediment: prolonged anaerobic resp.?

Extensive lacunal system in mesophyll continuous through petiole, stem & rhizomes/roots provides channel for O₂ diffusion, aided by Ps O₂ evolution during day.

Emergent hydrophytes transpire freely, esp. during day; some also show guttation.

Depending on spp., aerenchyma near the water sfc. formed from either the vascular or cork cambium (latter = phellogen); basically, aerenchyma seems to be a specialized form of cork

Organs possessing aerenchyma often lack an extensive lacunar system, so may serve as O₂ storing tissue and/or for floatation; seems NOT to be respiratory tissue. Sculthorpe Fig. 3.3

4. Effects of submergence

Although variable w/ spp., often submersed leaves are longer and narrower than emergent leaves of the same sp. Sculthorpe Figs. 3.5, 3.6

Other, mainly quantitative, alterations frequently observed upon submersion include:

more erect stems, longer internodes, sparser branching and root development

decreased leaf thickness, hairiness (glabrous = without hair) & Chl content

increased non-photosynthetic pigments (anthocyanins, anthoxanthins)

increased spongy relative to palisade mesophyll thus increased air spaces

development of chloroplasts in epidermis and altered stomatal distribution

decreased xylem & supportive tissue and increased lacunar development in submersed stems

FLOATING LEAVES: STRUCTURE & PHYSIOLOGY

Strong selective pressures on floating leaves have resulted in striking parallel evolution in all plants of this type regardless of phylogeny.

Major problem is physical stress of wind, waves, rain: favors leathery, circular, entire, peltate (attached in center) leaves w/ long flexible petioles and collenchyma-reinforced primary veins, more or less evident in various families; still, most floating-leaved plants are restricted to relatively calm and shallow waters. Sculthorpe Figs. 4.1, 4.2

Another unique problem = exposure to both air & water: thick cuticle and functional stomata only on upper sfc. for gas exchange (sporadic relict stomata on lower sfc.); upper palisade mesophyll w/ extensive lacunate spongy mesophyll of columnar cells; latter aids buoyancy while structural tissue adds strength. Sculthorpe Figs. 4.8, 4.9A

With a 2-dimensional space to array leaves, competition for light is intense: long petioles or stems permit spreading of leaves to avoid overlap, which nevertheless occurs in dense stands.

Petioles rapidly resume growth (both cell division & elongation) if leaves become submerged.

The causal factor(s) of growth cessation at the air/water interface are not known, and occasionally some spp. (Nuphar, Nymphaea) will produce emergent leaves in dense stands.

Floating leaf primordia from a benthic rhizome may take years to develop, and early development in water again may be hypoxic and require high osmotic pressure in the cells.

Typically lack vessels, having only tracheids in xylem.

Stranded plants of emersion-tolerant spp. form close rosettes of reduced leaves w/ fewer stomata and infurled margins.

SUBMERGED ORGANS

Morphology

Greatly decreased structural support tissue, unlignified, us. w/ little or no collenchyma/sclerenchyma; greatly reduced mesophyll w/ little dorsiventral differentiation due to random orientation of leaves toward light Sculthorpe Fig. 5.10

Extremely thin cuticle and leaves, and presence of chloroplasts in epidermis (sometimes more than in mesophyll, e.g. spp. of Ceratophyllum, Myriophyllum, Potamogeton), mimic terrestrial shade leaves, and probably reflect low PFD rather than adaptations to aquatic existence per se.

Two major growth forms based on differing apical organization:

1. flat, broad shoot apex: short axis producing rosette of radical leaves (directly from roots; e.g. Isoetes)

2. long, narrow shoot apex: elongated flexuous stem covered w/ leaves and rooted (sometimes sparsely) from nodes (e.g. Elodea)

leaves either alternate (e.g. Potamogeton), paired/opposite (e.g. Cabomba) or whorled (e.g. Elodea, Myriophyllum)

sympodial growth (no dominant shoot axis), older tissues dying

axillary buds may occur at every leaf or only at intervals of 2-12 nodes (spp.-dependent)

Three main types of leaves, only two of which are common:

1. entire: most common in both mono- and dicots of all habitats and climate zones

us. thin, translucent, very elongate (esp. in rosette spp.)

variously filiform or setaceous (e.g. Potamogeton pectinatus), small linear (e.g. Elodea) or ribbon-like (e.g. P. spp., Sagittaria spp.); and flat, undulate or bullate according to spp.

2. dissected: common in many dicots from tropics to temperate waters

many free segments radiating from petiole, w/ various orders of dissection and length/thickness depending on sp. and conditions

putative benefit of increased SA:V ratio relative to entire leaves not definitively demonstrated

Physiology

Gas diffusion through water is 100-1000x slower than in air, and PFD is reduced, so SA:V and boundary layers are important factors in hydrophyte physiology.

Typical anatomical features, e.g. thin leaves, cell walls & cuticle, and extensive air spaces facilitate dissolved gas and ion diffusion to/from cells

HCO₃^- as well as CO₂ can be used for photosynthesis in many submerged macrophytes Spence & Maberly Tables III, IV, VI, VII

Note: [C_t] = [DIC] = [CO₂ (aq)] + [HCO₃^-] + [CO₃^2-] (HCO₃^- us. dominates)

Alk = [HCO₃^-] + 2[CO₃^2-] + [OH^-] - [H⁺]

C_t/Alk = quotient of final [C_t] over alkalinity in a pH-drift expt. A low value means little C_t remains after Ps uptake, a high value means most C_t in sol'n is photosynthetically unavailable to the plant.

Lacunar [O₂] can be very high during daytime Ps, or very low at night, presenting interesting physiological challenges.

HETEROPHYLLY

Heterophylly is a complex phenomenon in which an individual plant has more than one kind of leaf, w/ each leaf form having some or all of the characteristics indicated above. Riemer Figs. 9.4, 9.5; Sculthorpe Figs. 8.10, 8.13

Present to a limited extent in some terrestrial plants, but is much more widespread and exaggerated among aquatic plants.

Depending on spp., the various leaf forms may arise in a fixed developmental sequence, and/or may occur opportunistically, e.g. when a submersed plant becomes emersed w/ decreasing water level.

Failure of many systematists to distinguish pheno- vs. genotypic plasticity has led to much taxonomic confusion because of irregular heterophylly, hence the reliance on floral characters for definitive identification (though even these are variable).

Differences between leaf types may be physiological as well as morphological.

Exogenous factors that may trigger development of floating or aerial leaves from a submerged stem upon nearing the surface probably vary w/ spp., and include low atm. [CO₂], high temp., long photoperiods, and the far red to red ratio.

In many cases the floating/aerial leaves develop while still underwater, so endogenous factors may be important, e.g. carbohydrate or nutrient status of the shoot meristem.

UNDERGROUND ORGANS

Two principal functions: anchorage and nutrient absorption.

Two major problems: substrate instability (silting and erosion) and anoxia (along w/ high CO₂, CH₄, and S^2-).

Substrate instability "solved" by vigorous rhizomes & fibrous adventitious roots to bind sediment particles.

Rhizome/root network in emergents may equal or even exceed (by up to 4x in Phragmites communis and Equisetum fluviatile) the biomass of aerial foliage; varies w/ local conditions Wetzel Table 18-3

Rhizome morphology and growth rate varies widely among spp.

Profuse root hairs typical in emergents & floating-leaved hydrophytes.

Smaller hydrophytes (e.g. Myriophyllum, Potamogeton, Marsilea) tend to have more delicate, but profusely branched rhizomes w/ matted fibrous roots; adventitious roots often spirally coiled (to aid anchorage?).

Rosette hydrophytes w/ radical leaves have variable root morphology, ranging from slender/soft to bulbous/succulent or hard/cormlike, and many spread by stolons.

Extensive lacunae in roots and/or rhizome (continuous w/ that of stems) are common but not universal in all hydrophytes.

O₂ gradient from leaves to roots drives downward O₂ diffusion to support aerobic respiration. Darkness at night or low PFD in dense stands, and high temp. decreases O₂ supply. Root growth typically inhibited in low [O₂], but some actually grow best in very low O₂!

Some emergents may at times transport enough O₂ to oxygenate the surrounding sediments, and some tolerate anaerobic respiration (producing ethanol) during prolonged anoxia.

It has been argued that lacunae also may increase mechanical strength w/ no additional respiratory demand.

In emergents it is widely agreed that the roots provide most of the water and nutrients to the entire plant. The relative importance of roots vs. shoots in nutrient uptake is less clear in floating-leaved and submerged spp., but at least N & P are probably obtained mainly from sediments in these plants also. This is an important area of uncertainty since it determines the extent to which macrophytes are sources vs. sinks of nutrients to the water column.

Barko et al. (1991. Aquat. Bot. 41:41-65) reviewed this topic and concluded that submersed macrophytes both control and are controlled by sediment nutrients. Shifts in spp. composition often reflect changing sediment properties. They present the following generalization of primary sources of specific nutrients to submersed macrophytes:

sediment: N, P, Fe, Mn, trace metals: these elements may thus be "pumped" by macrophytes from sediments to water column upon senescence/decomposition.

water column: Ca, Mg, Na, K, SO₄, Cl

Moreover, they conclude that most submersed macrophytes are not likely to be P-limited (!) because of the high exchangeable [P] in most, but not all, lake sediments; N limitiation is sometimes evident.

Barko & Smart (1986. Ecology 67:1328-1340.) demonstrated a positive relationship between sediment density (which is an inverse curvilinear function of organic content) and macrophyte (Hydrilla, Myriophyllum) growth. The effect seems to be indirect: sed. density determines nutrient diffusivity and availability.

FREE-FLOATING PLANTS

The free-floating (pleustonic) habit is very common and diverse, including some of the most widespread and abundant hydrophytes (e.g. Lemna, Salvinia, Pistia).

Morphology ranges from the tiny and simple Wolffia to elaborate rosette forms such as Eichhornia crassipes (water hyacinth, a monocot) and Trapa natans (a dicot), most but not all having dangling roots.

Evolutionary trend toward reduction in pteridophytes (Azolla, Salvinia), monocots (e.g. Lemnaceae) and dicots (e.g. Utricularia, which lacks roots); rosette spp. more primitive.

Some spp. of Lemnaceae, Utricularia, etc. "float" below sfc. but still produce aerial flowers.

Generally restricted to sheltered habitats, incl. slow-flowing rivers; all nutrition must come from water, so largely restricted to high ionic strength/high nutrient waters.

Rosette species

Stoloniferous and can form dense mats rapidly.

Perennial and free-floating except for early seedling development.

Variably dorsiventrally differentiated leaves, depending on floating vs. emergent habit.

Extensive lacunate mesophyll to aid floatation: Sculthorpe Fig. 7.8

Leaf rigidity largely effected by turgor, w/ little lignification.

Leaves are generally glabrous (nonhairy), but rainwater repelled by cuticle.

Invariably well-developed adventitious roots w/ both laterals & epidermal hairs, unusual among submerged plants; us. no anchorage role, rather may stabilize rosette; may develop Chl

Reduced Forms Sculthorpe Figs. 7.13, 7.15

Azolla & Salvinia (pteridophytes) and the Lemnaceae are greatly reduced in size (generally <1 mm to 1 cm) and complexity.

Most spp. of Lemna (1 root per thallus) & Spirodela (several roots) occur as colonies of 3⁺ thalli, easily broken up by rain, waves, animals, so rarely >2 generations of thalli joined.

The morpho-developmental origin of the reduced structures in the Lemnaceae is debated, i.e. are they reduced stems, leaves or something else?

Lemnaceae are only a few cell layers thick w/ large intercellular air spaces and chloroplasts in all cells.

Surface-floating spp. of Lemna & Spirodela have stomates and thick cuticle on top; stomates variably abundant in Wolffia & Wolffiella.

No vascular tissue in Wolffia & Wolffiella; reduced, nonlignified, vascular bundles present in Lemna & Spirodela

Roots, where present, are sparse & simple, often lacking vascularization and sometimes w/ Chl; root development in L. minor is regulated by the thallus carbohydrate/protein ratio.

L. minor thalli live for up to 5-6 weeks under ideal conditions, w/ decreased area in each successive generation from a given mother thallus due to fewer cells; each gives rise to a finite, clone-specific number of daughter thalli.

REPRODUCTION IN AQUATIC ANGIOSPERMS

Sexual Reproduction

Unlike algae that have evolved entirely in water and generally have motile gametes (at least the male) and often sex attractants, hydrophyte angiosperms have evolved varying degrees of modification of terrestrial modes of pollination.

Vast majority of hydrophytes have retained aerial pollination aided by wind (anemophilous) or insects (entomophilous), with vegetative morphological adaptations to support flowers in air (aerial flowers typically not specially adapted). Most spp. pollen dies in the water.

Only the permanently submerged FW genera Althenia, Ceratophyllum, Najas, Zannichellia, Ruppia, and some spp. of Callitriche have developed true hydrophily involving underwater pollination. Hydrophily appears universal in marine genera.

True hydrophilous spp. tend to have elongate, rodlike pollen that are theoretically more likely than sperical pollen to intercept a stigma while randomly drifting in or on the water (Cox. Sci. Amer. 10/93). A similar prediction is made for elongate stigmas, which is consistent w/ the shape in many spp.

Traditional dogma was that hydrophilous plants rarely flower or pollinate. Cox (Sci. Amer. 10/93) refutes this, and Riemer (1984. Intro. to FW Vegetation) argues that many spp. may do both but rarely produce viable seeds, and even those that do still rely more heavily on vegetative reproduction.

Whereas <5% of terrestrial plants are dioecious, >50% of hydrophytes are dioecious, presumably to eliminate the otherwise high probability of selfing in water.

Populations of FW, truly hydrophilous spp. are presumably reproductively isolated, so interpopulation gene flow must be by vegetative fragments or fruits. Many hydrophilous populations are indeed genetically homogeneous, but extensive clonal growth cannot be ruled out as a contributing or primary cause.

Some specific FW examples exhibit the diversity of strategies for specialized aquatic sexual reproduction:

1. Vallisneria (wild celery) and Lagarosiphon (African pondweed): Both have submerged male flowers that abscise and float to surface, then raft w/ emergent stamens until colliding w/ or falling into floating female flowers (pollen intolerant of wetting). Sculthorpe Fig. 9.18M-R

2. Hydrilla verticillata: Similar to (1) except pollen grains, intolerant of wetting, are catapulted from male flowers at the sfc.; only low % direct hits on nearby female flowers are successful.

3. Ceratophyllum demersum: Male flowers produced submerged, then released to drift on surface, releasing pollen that rain down on submerged female flowers (true hydrophily). DNA evidence now indicates that this sp. May be evolutionarily ancient, possibly not even from a terrestrial ancestor (see Raven et al. 1999, p. 526) Sculthorpe Fig. 9.21C-G

4. Najas spp.: Another true hydrophile w/ submerged flowers & pollination. Most spp. are monoecious, w/ greatly reduced floral morphology: male flowers = single sessile anther on a short stalk, female flowers = single sessile carpel us. w/ 2 stigmas. Sculthorpe Fig. 9.17

5. Lemnaceae: Greatly reduced aerial flowers rarely (never in some spp.) observed

Lemna & Spirodela inflorescence = 1 female (1 sessile carpel w/ concave stigma) & 2 male (each 1 stamen) flowers in a marginal pocket near the thallus base.

Wolffia & Wolffiella inflorescence = 1 female (1 sessile carpel) & 1 male flower (1 stamen) in a furrow on upper surface. Sculthorpe Fig. 9.12

Factors suspected to influence the onset of flowering include carbohydrate reserves (thus PFD), nitrogen, photoperiod (phytochrome response), temperature, and metals. Definitive experiments are spotty so no generalizations are possible. Most temperate hydrophytes tend to flower in summer and are presumably long-day plants, but there are differences even within a genus, as in Lemna where L. gibba is a long-day and L. perpusilla a short-day species.

Vast majority of aerial-pollinating hydrophytes, except for emergent spp., tend to have post-fertilization mechanisms for submerged fruit development, e.g. by curvature (e.g. Cabomba, Eichhornia, Nymphaea, Potamogeton) or spiral retraction (e.g. Vallisneria, Enhalus, Ruppia) of the peduncle or carpel stalk.

Vast majority also exhibit prolonged seed dormancy, in some cases retaining viability for at least several decades (e.g. the lotuses Nelumbo spp.); presumably this increases the range of dispersal by either water- or animal-mediated transport, as well as permitting germination under ideal conditions.

Vegetative Reproduction & Perennation

In general, reproduction is thought to be primarily (exclusively for many pops. of e.g. Elodea canadensis) vegetative in hydrophytes; extensive clonal growth may function as a large photosynthate/nutrient sink which deters the onset of flowering, as in many terrestrial plants, but photoperiod and/or temperature triggers may override this tendency.

May be easiest solution to propagation since most spp. are ill-adapted for aquatic pollination, and most vegetative propagules seem to be efficiently if "accidentally" transported by floods, waterfowl, other animals and humans, even to e.g. small, isolated high altitude lakes.

There appears to be an evolutionary trend toward replacement of sexual by vegetative reprod., and there are many intermediate examples:

Submerged individuals of amphibious spp. are usually sterile.

Cleistogamous flowers, self-fertilization in bud, rare incidence of flowering, and scarcity of viable seeds are not uncommon.

Pseudovivipary is common, as in development of an apical turion if inflorescence is submerged in Myriophyllum verticillatum, or young plantlets in place of sporangia in deep water Isoetes plants.

Vegetative propagules of hydrophytes are very similar in morphology & development to those of terrestrial herbs:

Regeneration from any bud-containing plant fragment is common in reduced floating spp. and delicate/long-stemmed submerged spp.

Gemmipary, the development of young plantlets (bulbils) from vegetative buds borne on the parent plant, occurs in several groups, either normally or upon wounding.

Rhizomes, stolons (underground) and runners (aboveground) are widespread among all growth forms, functioning as a colonization mechanism, and sometimes in food reserve storage or as resistant perennation organs. Sculthorpe Fig. 10.6

Toward the end of the growing season (or in dry season in tropical spp.), some temperate hydrophytes produce winter-dormant perennation tubers (e.g. Potamogeton, Saggitaria), turions (dwarf shoots; e.g. Myriophyllum, Potamogeton), or heavily cuticularised dense shoot apices (e.g. Ceratophyllum, Elodea). Sculthorpe Fig. 10.9; Hutchinson III Fig. 93

The triggering mechanism(s) for formation and later germination of perrenating, dormant "hibernacula" are not well studied, but decreased photoperiod, temperature, water, and perhaps PFD & nutrients are possible factors in formation, and mainly PFD and temperature in germination.

MACROPHYTE ECOLOGY AND DISTRIBUTION

The following is based largely on Hutchinson Vol. III and Sand-Jensen & Borum (1991. Aquat. Bot. 41:137-175).

The dominance of macrophyte productivity in many shallow lakes and streams is well established but not yet frequently acknowledged in management & ecosystem models. Rates of Ps and R_d can exceed those in the water column 100-fold for macrophytes and 10,000-fold for benthic microalgae.

Water hardness is a major lake chemistry factor influencing the type of macrophytes in temperate zone lakes. Hutchinson III Table 62

Water hardness is a complex, multifaceted problem; it is difficult to ascribe causality to any 1 of several correlated factors related to hardness: soft waters are low in all dissolved salts, pH and often nutrients, and may have more alkalies (Na, K) than alkaline earth (Ca, Mg) elements

pH probably plays a role in soft waters, e.g. Potamogeton spp. absent where pH is chronically <6, regardless of [Ca]; there are also clearly facultative brackish spp. such as P. pectinatus.

The deepest reported tracheophyte growth is ~11 m in Lake Titicaca, w/ ~9 m a more typical limit in clear lakes; bryo-/charophytes may extend ~10x deeper. Certain spp. routinely reach ~2% Io depth, e.g. P. praelongus, P. bertchtoldii, P. obtusifolius, Ceratophyllum demersum.

The depth limit is certainly related to PFD (and thus water clarity), but pressure may also be a factor (2 atm @ 10 m) for tracheophytes (because of lacunae?).

In many Canadian lakes, the lower depth boundary of rooted plants corresponded to ~20% I_o, or ~5 mol photons m^-2 d^-1 (averaging ~100 umol photons m^-2 s^-1 in a 12:12 LD cycle); as expected, this is considerably higher than I_c for short-term Ps.

Sediment properties (grain size; nutrient, O₂ & organic content) are other possible major factors, an area of active research.

The maximum specific growth rate (u_max) among phytoplankton, macroalgae and rooted macrophytes are allometrically related to SA/V ratio: log u_max (d^-1) = 0.66 log SA/V (m^-1). Differences among groups in net u in the field are generally smaller than in lab studies because phytoplankton are more nuts limited than benthic plants. Sand-Jensen & Borum Table 3

"The nutrient requirements of rooted macrophytes are lower than those of microalgae because of low growth rates, high internal C:N:P ratios and the existence of nutrient conserving mechanisms, and nutrient limitation is less important because the plants exploit the rich nutrient pools of the sediment." Sand-Jensen & Borum Fig. 1, Table 2

"The phototrophs compete for light, nuts and [DIC], and the balance among phototrophs changes with size, depth and nutrient richness of the ecosystem. Phytoplankters dominate in deep lakes and… may also, together w/ periphyton, dominate nutrient-rich shallow waters because of shading effects on macrophytes and bemthic microalgae. However, shallow lakes… with low nuts availability in the water column are dominated by benthic phototrophs because of their lower nuts requirements and contact to sediment nuts pools."

Light is expected to be much more limiting (and nuts less limiting) to rooted macrophytes than to phytoplankton. Sand-Jensen & Borum Figs. 3, 4, Table 4

Because of this, the consistent P-Chl relationship observed in lake phytoplankton should not be uncritically expected to apply to benthic phototroph communities.

Exposure to waves or currents precludes establishment of rooted macrophytes due to either physical damage or unsuitable stony substrata. Those that tolerate exposed lake conditions tend to be small, stiff-leaved rosette spp., and conversely sheltered habitats are dominated by macrophytes w/ apical growth, and highly branched long stems.

Some untested models have been suggested to describe the relative contribution of rooted macrophytes to primary productivity in lakes:

rel. P_mac = (Z_mean/Z_max)(L/L_o)/P_tot

OR rel. P_mac = (Z_secchi/Z_mean)(L/L_o)

where L and L_o = shoreline length of actual vs. circular lake (same area)

"Across large plant patches there may be a change in the potentially growth-limiting conditions from physical stress at the edge to light or diffusive limitation… in the middle". Growth in patch size will mainly occur in the direction of lower physical disturbance.

Although growth rates of macrophytes are known in many cases, colonization and loss processes have been poorly studied, severely restricting predictive/management capabilities.

"Typical" seasonal biomass/productivity trends of annuals shown in: Wetzel Figs. 18-5, 6

Generalized trends in spp. dominance through lake fill-in, based on only a few well-studied lakes and much intuitive guesswork, are shown in Sculthorpe Table 12.1 These changes often VERY slow, occasionally rapid in areas of heavy siltation (deltas).

Productivity and max. biomass of macrophytes varies greatly. Wetzel Tables 18-5, 6

Macrophyte Roles in Aquatic Ecosystems

Most lakes and ponds are semi-closed w/ respect to food webs, so aquatic communities are largely self-supporting, i.e. dependent on aquatic primary producers.

Compared to phytoplankton, macrophytes in general are believed to be relatively minor contributors to fisheries food webs; they may actually allow greater survisvorship of fish because they provide shelter from predation, but favor larger populations of smaller fish.

Macrophytes are preferred food of many waterfowl (e.g. >85% of diet of green-winged teals), which may control macrophyte biomass. Not all spp. are equal: pondweeds (Potamogeton) are heavily grazed while Myriophyllum and Cabomba are largely avoided. Depending on spp., either the whole plant (Lemnaceae) or only selected parts (e.g. tubers or seeds) are eaten

In dense stands in calm waters, macrophytes may significantly affect the [O₂]: increase O₂ by Ps, decrease O₂ by R_d, decomposition of scenescent tissue, and shading of phytoplankton & submerged plants (thus increased R_d) by floating macrophytes. Sculthorpe Figs. 12.9, 12.10

Macrophytes provide a > surface area for colonization by leeches, snails, insects, protozoans, and they support much larger populations per unit surface area, compared to an unvegetated substrate. Many of these fauna control the epiphyte biomass. Spp. differ greatly as substrata for invertebrates, perhaps related to variable protection from predation. Riemer Table 8.1

Macrophytes increase the rate of natural fill-in of lakes, both directly by their death and incomplete decomposition and indirectly by slowing water currents which speeds siltation.

Dense stands of floating or aerial leaves increase evaporative loss from lakes by up to 3-4x.

Azolla, which harbors N₂-fixing Anabaena azollae, can greatly increase the [N] in a lake/pond, and has been used to N-enrich rice fields for centuries in the orient.

Spp. assemblages dominated by e.g. Najas, Zannichellia, and certain spp. of Potamogeton tend to be relatively insensitive to external disturbances, whereas nymphaeids (incl. Nuphar, Brasenia, etc. and floating-leaved spp. of Potamogeton, e.g. P. nodosum) tend to form communities dependent on the dominant species thus more sensitive to disturbance.

Dense macrophyte stands in rivers may occupy 1-4% (rarely up to 10%) of the total river volume, and may decrease flow rate locally by up to 25% and increase water level by up to 70-80 cm.