Nutrient-poor Sediments Research Articles

Effects of benthic fish and light regimes on water quality and the growth of Vallisneria natans with two sediment types.

In shal low eutrophic lakes, submersed macrophytes are essential for maintaining a clear water state and they are significantly affected by benthic fish disturbance, light availability, and sediment types. We conducted a mesocosm experiment with benthic fish (Misgurnus anguillicaudatus), two light regimes, and submerged macrophyte (Vallisneria natans) growing in two sediment types to investigate the ecological effects of benthic fish and light regimes on water quality and the growth of submersed macrophyte. Our findings indicated that the benthic fish increased the concentrations of total nitrogen, total phosphorus, and total dissolved phosphorus in the overlying water. The effects of benthic fish on ammonia-nitrogen (NH4+-N) and chlorophyll a (Chl-a) contents were related to light regimes. Fish disturbance indirectly promoted the growth of macrophytes growing in sand by increasing NH4+-N content in overlying water. However, the increasing Chl-a content stimulated by fish disturbance and high light regime reduced the growth of submersed macrophytes growing in clay due to shading. Macrophytes with different sediments had different strategies coping with light. Plants growing in sand responded to low light mainly by adjusting the leaf and root biomass allocation, whereas plants growing in clay responded to low light by physiologically adjusting the soluble carbohydrate content. The findings of this study might help restore lake vegetation to some degree, and using nutrient-poor sediment might be an appropriate method to avoid the detrimental effects of fish-mediated disturbances on the growth of submerged macrophytes.

Interactive Effects of Rising Temperature and Nutrient Enrichment on Aquatic Plant Growth, Stoichiometry, and Palatability.

The abundance and stoichiometry of aquatic plants are crucial for nutrient cycling and energy transfer in aquatic ecosystems. However, the interactive effects of multiple global environmental changes, including temperature rise and eutrophication, on aquatic plant stoichiometry and palatability remain largely unknown. Here, we hypothesized that (1) plant growth rates increase faster with rising temperature in nutrient-rich than nutrient-poor sediments; (2) plant carbon (C): nutrient ratios [nitrogen (N) and phosphorus (P)] respond differently to rising temperatures at contrasting nutrient conditions of the sediment; (3) external nutrient loading to the water column limits the growth of plants and decreases plant C:nutrient ratios; and that (4) changes in plant stoichiometry affect plant palatability. We used the common rooted submerged plant Vallisneria spiralis as a model species to test the effects of temperature and nutrient availability in both the sediment and the water column on plant growth and stoichiometry in a full-factorial experiment. The results confirmed that plants grew faster in nutrient-rich than nutrient-poor sediments with rising temperature, whereas external nutrient loading decreased the growth of plants due to competition by algae. The plant C: N and C: P ratios responded differently at different nutrient conditions to rising temperature. Rising temperature increased the metabolic rates of organisms, increased the nutrient availability in the sediment and enhanced plant growth. Plant growth was limited by a shortage of N in the nutrient-poor sediment and in the treatment with external nutrient loading to the water column, as a consequence, the limited plant growth caused an accumulation of P in the plants. Therefore, the effects of temperature on aquatic plant C:nutrient ratios did not only depend on the availability of the specific nutrients in the environment, but also on plant growth, which could result in either increased, unaltered or decreased plant C:nutrient ratios in response to temperature rise. Plant feeding trial assays with the generalist consumer Lymnaea stagnalis (Gastropoda) did not show effects of temperature or nutrient treatments on plant consumption rates. Overall, our results implicate that warming and eutrophication might interactively affect plant abundance and plant stoichiometry, and therefore influence nutrient cycling in aquatic ecosystems.

Eutrophic water or fertile sediment: which is more important for the growth of invasive aquatic macrophyte Myriophyllum aquaticum?

Invasive macrophyte Myriophyllum aquaticum is capable of assimilating nutrients from both the sediments and the water column. However, which is the major source of nutrients supporting M. aquaticum growth under various nutrient regimes has not been well documented. Here we conducted a two-factorial outdoor experiment (three levels of nutrient loading and two types of sediments) from 23 May to 27 June 2016, to assess M. aquaticum responses to different nutrient levels in the water column and the sediment. Results showed that concentrations of total nitrogen, total phosphorus and chlorophyll a in the water column increased in the treatment groups, but decreased slightly in the control group (nutrient-poor sediment and no nutrient addition). Sediment type had a significant effect on the growth M. aquaticum, while there were no significant effects of nutrient loading levels and the interactions between the two factors. Mean relative growth rate, mean plant height, mean stem diameter, the number of lateral branches and roots in the nutrient-rich sediment treatments were 1.6, 1.2, 1.6, 3.2 and 5.9 folds greater than in the nutrient-poor sediment treatments, respectively. These results suggest that nutrient-rich sediment has a positive effect on the growth of M. aquaticum. This study reveals possible expansion mechanism of the exotic M. aquaticum in China, and may provide valuable information on the ecological restoration of eutrophic freshwaters.

AM fungal communities inhabiting the roots of submerged aquatic plant Lobelia dortmanna are diverse and include a high proportion of novel taxa.

While the arbuscular mycorrhizal (AM) symbiosis is known to be widespread in terrestrial ecosystems, there is growing evidence that aquatic plants also form the symbiosis. It has been suggested that symbiosis with AM fungi may represent an important adaptation for isoëtid plants growing on nutrient-poor sediments in oligotrophic lakes. In this study, we address AM fungal root colonization intensity, richness and community composition (based on small subunit (SSU) ribosomal RNA (rRNA) gene sequencing) in five populations of the isoëtid plant species Lobelia dortmanna inhabiting oligotrophic lakes in Southern Sweden. We found that the roots of L. dortmanna hosted rich AM fungal communities and about 15% of the detected molecular taxa were previously unrecorded. AM fungal root colonization intensity and taxon richness varied along an environmental gradient, being higher in oligotrophic and lower in mesotrophic lakes. The overall phylogenetic structure of this aquatic fungal community differed from that described in terrestrial systems: The roots of L. dortmanna hosted more Archaeosporaceae and fewer Glomeraceae taxa than would be expected based on global data from terrestrial AM fungal communities.

Pyrosequencing analysis of microbial communities reveals dominant cosmopolitan phylotypes in deep-sea sediments of the eastern Mediterranean Sea

The deep eastern basin of the Mediterranean Sea is considered to be one of the world's most oligotrophic areas in the world. Here we performed pyrosequenicng analysis of bacterial and archaeal communities in oxic nutrient-poor sediments collected from the eastern Mediterranean at 1025–4393 m depth. Microbial communities were surveyed by targeting the hypervariable V5–V6 regions of the 16S ribosomal RNA gene using bar-coded pyrosequencing. With a total of 13,194 operational taxonomic units (OTUs) or phylotypes at 97% sequence similarities, the phylogenetic affiliation of microbes was assigned to 23 bacterial and 2 archaeal known phyla, 23 candidate divisions at the phylum level and distributed into 186 families. It was further revealed that the microbial consortia inhabiting all sampling sites were highly diverse, but dominated by phylotypes closely related to members of the genus Pseudomonas and Marine Group I archaea. Such pronounced and widespread enrichment probably manifests the cosmopolitan character of these species and raises questions about their metabolic adaptation to the physical stressors and low nutrient availability of the deep eastern Mediterranean Sea.

Influence of sediment on the growth of the invasive macrophyte Najas marina ssp. intermedia in lakes

Najas marina ssp. intermedia (Wolfg. ex Gorski) Casper ( Najas intermedia ) is a thermophile macrophyte species native to Central Europe, which started mass spreading across Southern Germany about 10 years ago, almost reaching the intensity of the invasive neophyte Elodea nuttallii . As part of a study to examine the spread of N. intermedia with regard to climate change, predominant populations of N. intermedia were continuously monitored. The observations revealed that some areas and lakes remained free of N. intermedia , although water temperatures and light conditions were similar to locations with predominant populations of N. intermedia . As a result, growth experiments were conducted to show that the properties of the lake sediment can affect the growth of N. intermedia . Four lakes without populations of N. intermedia and with different environmental conditions were chosen as experimental sites. During the experiments eight different sediments from four different lakes were used. Five sediments were collected from sites with extensive or predominant Najas populations, three sediments originated from locations with no or minor amounts of N. intermedia . The experiments revealed that the sediment from different lakes as well as from different locations within the same lake significantly differs in nutrient concentration and density and that those differences can affect the growth of N. intermedia . Plants growing in nutrient-rich sediment (SRP: 15 mg 100 g −1 soil, Total-P: 50 mg 100 g −1 soil) reached with 43.1 (±6.9) mm day −1 , compared to 6.9 (±2.0) mm day −1 in sediment with lower nutrient concentrations (SRP: 2 mg 100 g −1 soil, Total-P: 10 mg 100 g −1 soil), higher growth rates and were less affected by the density of the sediment. The density of the sediment, on the other hand, played a significant role under conditions with low nutrient concentrations, respectively when sediments with similar nutrient concentrations were compared. For example, a comparison of sediments with a soluble phosphor concentration of 2 mg 100 g −1 soil and a total phosphor concentration of 10 mg 100 g −1 soil showed that plants growing in sediment with 75.7% particles > 0.063 mm reached twice the growth rates (6.9 ± 2.0 mm day −1 ) than plant growing in sediment with only 47% particles >0.063 mm (1.7 ± 0.6 mm day −1 ). Nutrient-poor sediment (SRP: <1 mg 100 g −1 soil, Total-P: <1 mg 100 g −1 soil), inhibited the growth of N. intermedia (0.8 ± 0.2 mm day −1 ). The significantly different growth rates of N. intermedia show that the lake sediment, respectively the nutrient concentration and density must be included in assessments and models regarding the growth and spread of N. intermedia .

Influence of environmental factors on the spatial distribution and diversity of forest soil in Latvia

This study was carried out to determine the spatial relationships between environmental factors (Quaternary deposits, topographical situation, land cover, forest site types, tree species, soil texture) and soil groups, and their prefix qualifiers (according to the international Food and Agricultural Organization soil classification system World Reference Base for Soil Resources (FAO WRB)). The results show that it is possible to establish relationships between the distribution of environmental factors and soil groups by applying the generalized linear models in data statistical analysis, using the R 2.11.1 software for processing data from 113 sampling plots throughout the forest territory of Latvia. A very high diversity of soil groups in a relatively similar geological structure was revealed. For various reasons there is not always close relationship between the soil group, their prefix qualifiers and Quaternary deposits, as well as between forest site types, the dominant tree species and specific soil group and its prefix qualifiers. Close correlation was established between Quaternary deposits, forest site types, dominant tree species and soil groups within nutrient-poor sediments and very rich deposits containing free carbonates. No significant relationship was detected between the CORINE Land Cover 2005 classes, topographical situation and soil group.

Turion production and nutrient reserves in Potamogeton crispus are influenced by sediment nutrient level

The influence of sediment nutrient content on asexual propagule production in plants is poorly understood, especially in submersed macrophytes. To improve the understanding of turion (an aboveground asexual propagule) production, Potamogeton crispus L. was planted in 2 experimental conditions that differed in their levels of sediment nutrients. After 10 wk of growth, sediment nutrient level had significantly impacted the plants' vegetative and reproductive traits. Most vegetative trait measures of P. crispus (e.g. leaf mass fraction and stem mass fraction) were higher when plants were grown in nutrient-rich sediment compared with plants grown in nutri- ent-poor sediment. Reproductive trait measures (e.g. turion mass fraction and individual turion biomass) were higher in plants grown in nutrient-poor sediment compared with plants grown in nutrient-rich sediment. Plants grown in nutrient-rich sediment produced a larger number of small turions ( 100 mg) and stored more total nonstructural carbohydrate (the major proportion of which was starch) in them. Path analysis revealed that total plant biomass (strong positive effect), leaf and stem biomass (weak negative effects) had direct effects on total turion biomass, which consequently affected turion size and number. Moreover, ramet number and mean shoot height also had weak but direct effects (both negative effects) on turion size and number. These results demonstrate that sediment nutrient content mediates plant vegetative traits and can subsequently affect turion production and reserves in P. crispus.

Size-related auto-fragment production and carbohydrate storage in auto-fragment of Myriophyllum spicatum L. in response to sediment nutrient and plant density

Size-related asexual reproduction of submersed macrophytes is still poorly understood. Here, we investigate how size-related auto-fragmentation in Myriophyllum spicatum L. responds to sediment nutrients and plant density. An experiment was carried out with sediments containing two different nutrient levels and with two levels of plant density. The results show that sediment nutrients and plant density brought about a strong dependency of auto-fragment production and the amount of total non-structural carbohydrate (TNC) storage in auto-fragments on individual plant size (total plant biomass). However, these two factors acted differently on size dependency. Sediment nutrients positively affected auto-fragment production and the amount of TNC in auto-fragments of M. spicatum. High concentrations of sediment nutrients significantly increased these two traits in absolute value and the value relative to plant size. Although the auto-fragment biomass and the amount of TNC in auto-fragments did not differ between density treatments when plant size was considered, the absolute values of these two traits were much larger in the low plant density treatment than in the high plant density treatment, which suggested an indirect negative effect of plant density on the auto-fragmentation of M. spicatum. In addition, higher percentages of large auto-fragments (>100 mg) were produced by plants that grew in nutrient poor sediment and low plant density environment than plants in nutrient rich sediment and high plant density environment. These results do not solely highlight a size-dependent effect, but also a size-independent effect of auto-fragment production and the amount of TNC in auto-fragments of M. spicatum. Furthermore, such size-independent effects can be explained by the significant biomass partitioning differences and the similar TNC-concentrations in auto-fragments under different environmental conditions.

Asexual propagations of introduced exotic macrophytes Elodea nuttallii, Myriophyllum aquaticum, and M. propinquum are improved by nutrient-rich sediments in China

An increasing number of recent studies indicate that multiple interacting factors can affect the invasion of plants. However, few studies have focused on asexual propagation and the interaction of propagation with environmental factors that regulate the invasive potential of introduced exotic species in aquatic habitats. This study was designed to investigate the differences in asexual propagation between introduced exotic and non-invasive native aquatic macrophytes in nutrient-poor and nutrient-rich sediments and to test the hypothesis that differences in asexual propagation (stem fragment production) and propagule establishment between introduced exotic and non-invasive native macrophytes are driven by sediment nutrient levels. Three exotic aquatic macrophytes (Elodea nuttallii, Myriophyllum aquaticum, and M. propinquum) recently introduced to China and their non-invasive native counterparts (Hydrillaverticillata, M. oguraense, and M. ussuriense) were used for comparison in nutrient-poor (TN 0.59 and TP 0.03 mg g−1) and nutrient-rich (TN 2.35 and TP 0.10 mg g−1) sediments. After 8 weeks of growth, the exotic species tended to produce more total biomass, branch biomass and apical shoots and have higher relative growth rate (RGR) than their native counterparts in nutrient-rich sediment. Rooting efficiency and root growth of exotic fragments were higher than that of native counterparts in nutrient-rich sediment, although the survival rates of fragments did not differ between native and exotic species. In addition, superior traits (rooting efficiency and root growth) of exotic species were also observed in nutrient-poor sediment, but to a lesser degree than in nutrient-rich sediment. These results suggest that asexual propagation of these three introduced exotic macrophytes is more effective in nutrient-rich sediment than in nutrient-poor sediment in China.

The role of wind and splash erosion in inland drift-sand areas in the Netherlands

European drift-sand areas reached their maximum size in the 19th century. From the late 19th until the mid-20th century most drift-sand areas in Europe were successfully reforested. Today drift-sand formation can still be studied in the Netherlands. However, due to change in land-use, their limited size, eutrophication and colonisation by invasive exotics, the area occupied by active drift sand is rapidly declining and with it its unique fauna and vegetation. Between 2001 and 2005, experiments and observations were carried out at the inland drift-sand area Kootwijkerzand (central Netherlands) to know more about the extent, the intensity and the role of wind and splash erosion in a drift-sand ecosystem. Measurements were carried out on the horizontal sediment flux, the vertical accumulation flux, the raising and lowering of the surface, the organic matter content of the topsoil, the saltation activity and the weather conditions. An attempt was also made to evaluate the effect of various measures to re-activate erosion. Without sufficient erosion the scattered drift-sand vegetation will rapidly change into closed, grass-dominated vegetation. The role of erosion in the drift-sand ecosystem is therefore crucial: erosion slows down the succession rate because it causes the regular deposition of nutrient-poor sediment, and facilitates the regular renewal of the vegetation by creating new locations with extremely poor conditions for colonisation (blow-outs). On bare deflation plains wind erosion is the dominant process. Here the transport generated by splash drift can be of the same magnitude as during a moderate wind erosion event, but splash drift events are less frequent and also shorter than wind erosion events. They normally represent less than 10% of the total sediment transport. In the scattered drift-sand vegetation areas, on the other hand, splash erosion and splash drift dominate. Here the organic matter content of the transported (and later deposited) sediment is equal to the organic matter content of the parent soil, showing that splash (drift) has been the main transport mechanism. Biological crusts developed in these areas may later reduce the influence of splash. High transport rates are needed to reactivate wind erosion in vegetated areas. Under the present conditions the erosion activity in most drift-sand areas is small. Heavy wind erosion is only observed during extreme storms.

Sedimentation stress in a scleractinian coral exposed to terrestrial and marine sediments with contrasting physical, organic and geochemical properties

Terrestrial runoff increases siltation and nutrient availability on coastal coral reefs worldwide. However the factors determining stress in corals when exposed to short-term sedimentation, including the interactions between sediments and nutrients, are little understood. We exposed corals to ten different sediment types at environmentally relevant concentrations (33 to 160 mg DW cm −2) and exposure times (12 to 60 h) in laboratory and field experiments. The sediments originated from 2 estuaries, 2 nearshore and one offshore locations and also included ground-up aragonite. For two of these sediments, three grain size fractions were used (silt < 63 μm, fine sand: 63–250 μm, medium sand: 250–500 μm). Sediments were characterised by 19 parameters grouped into “physical”, “organic and nutrient-related” and “geochemical” parameters. Changes in the photosynthetic yield of the coral Montipora peltiformis was measured by pulse–amplitude modulated chlorophyll fluorometry (PAM) as proxy for photophysiological stress from exposure, and to determine rates of recovery. Different sediments exerted greatly contrasting levels of stress in the corals. Our results show that grain size and organic and nutrient-related sediment properties are key factors determining sedimentation stress in corals after short-term exposure. Photophysiological stress was measurable after 36 h of exposure to most of the silt-sized sediments, and coral recovery was incomplete after 48 to 96 h recovery time. The four sandy sediment types caused no measurable stress at the same concentration for the same exposure time. Stress levels were strongly related to the values of organic and nutrient-related parameters in the sediment, weakly related to the physical parameters and unrelated to the geochemical parameters measured. M. peltiformis removed the sandy grain size classes more easily than the silt, and nutrient-poor sediments were removed more easily than nutrient-rich sediments. Anoxia developed on the sediment surfaces of the nutrient-rich silts, which had become slimy and smelled of hydrogen sulphide, suggesting increased bacterial activity. Our finding that silt-sized and nutrient-rich sediments can stress corals after short exposure, while sandy sediments or nutrient-poor silts affect corals to a lesser extent, will help refining predictions of sedimentation threats to coral reefs at given environmental conditions.

The Aquatic Macrophyte Community of Onondaga Lake: Field Survey and Plant Growth Bioassays of Lake Sediments

ABSTRACT Onondaga Lake, located in the Syracuse metropolitan area of upstate New York, has been heavily impacted by domestic and industrial waste effluents, resulting in a lake with high salinity, low light availability, and a highly calcareous, nutrient-poor sediment. These factors appear interrelated in reducing the cover, distribution and diversity of aquatic plants between 1940 and 1990. A quantitative survey in 1991 found that only 13% of the littoral zone had any aquatic plants. The plant community was dominated by Potamogeton pectinatus (11%), with four other submersed aquatic plants found: Ceratophyllum demersum, Heteranihera dubia, Myriophyllum spicatum, and Potamogeton crispus. Aquatic plants were found less often than expected on the calcium-carbonate oncolite sediments, which are formed from precipitated calcium carbonate, compared to other sediment types in the lake. Laboratory studies were developed to evaluate the role of sediments in limiting plant growth. These studies showed that Onondag...

Flowering time as influenced by nitrate fertilisation in Ruppia drepanensis Tineo

In a laboratory experiment, Ruppia drepanensis Tineo seedlings from a brackish marsh in southern Spain were grown with and without potassium nitrate (1.4 g N m −2 week −1 added to the water column), both under nutrient-rich (clay:sand mixture) and nutrient-poor (sand) sediment conditions. Potassium nitrate fertilisation delayed flower initiation within both types of sediment, but it also resulted in increased flower abundance under nutrient-poor sediment conditions. Differences between plants in flower production were positively correlated with the above- to belowground biomass ratio. We thus propose that nutrient supply and biomass allocation should be incorporated in the age- and size-dependent models for plant reproductive effort. These models should also consider separately the induction of flowering and the production of flowers (flower abundance).

The relationship between P, Fe and Mn uptakes by submersed rooted angiosperms

Three species of submersed rooted aquatic angiosperms (Lagarosiphon major, Myriophyllum triphyllum and Potamogeton ochreatus) were grown on nutrient-rich and nutrient-poor sediments in Lake Taupo, an oligotrophic New Zealand lake. Results indicate that plants grown on nutrient-rich sediments had significantly higher shoot tissue concentrations of P, Fe and Mn than those grown on nutrient-poor. High correlations between all three nutrients were recorded for all species. This suggested that the availabilities of P, Fe and Mn in these sediments to these aquatic plants were interrelated.

Diel Pulses of O_2 and CO_2 in Sandy Lake Sediments Inhabited by Lobelia Dortmanna

Lobelia dortmanna is a common representative of the small isoetid plants dominating the vegetation on sandy sediments in nutrient—poor softwater lakes in North America and Europe. We report here the discovery of pronounced depth profiles and pulses of dissolved O2 and CO2 in the pore—water of vegetated sediments between light and darkness. Pulses of O2 and CO2 were extensive in the upper sediment layers of high root density, because Lobelia roots absorb CO2 and release O2 during photosynthesis but absorb O2 and release CO2 during respiration, whereas fluxes across leaf surfaces are impeded by a cuticle of low gas permeability. Release of most O2 from the roots during Lobelia photosynthesis leads to O2 concentrations close to saturation in the root zone and O2 penetration to great sediment depth because of low microbial O2 consumption rates. The CO2 concentrations in the root zone of the most nutrient—poor sediments examined here (0.37—0.71 mmol/L) were below the levels required to saturate Lobelia photosynthesis (&gt;3 mmol/L), and experimental enrichment with CO2 stimulated the development of higher O2 concentrations in the root zone. Cycles of O2, CO2 and nutrients are, therefore, intimately coupled in the Lobelia—sediment system, in contrast to terrestrial and other aquatic plants, where gas and nutrient fluxes are physically separated between leaves and roots and their respective surrounding media. The release of O2 from the roots should ensure efficient aerobic degradation or organic matter, lead to oxidized forms of Fe, Mn, and N, and can supply O2 to mycorrhizal fungi and an aerobic fauna associated with the roots.

A depauperate benthic assemblage from the nutrient-poor sediments of the Puerto Rico Trench

The Puerto Rico Trench contains a low-density-biomass assemblage of macrofauna and meiofauna. The depauperate character of this assemblage is primarily the result of the low percentage and refractory nature of organic matter in the sediments. Recent turbidite flows may have contributed to the paucity of fauna. Sediments from this portion of the Puerto Rico Trench are the result of pelagic sedimentation interspersed with thin laminae that probably originated as turbidity flows from the northern and eastern, or deep-sea, walls of the trench. These recently deposited pelagic clays are low in available organic matter. Bioturbation and other mechanical and geochemical processes have not compacted these sediments into a stable, habitable substrate. The “oligotrophic” assemblages found in the Puerto Rico Trench contrast with “eutrophic” hadal assemblages which inhabit coarser-grained, organic-rich sediments. These types of sediments usually originate as turbidity flows from the walls of island-arc trenches proximal to shallow-water shelves and provide a substrate of higher shear strength and greater nutritive value.

Microsensor Analysis of Oxygen in the Rhizosphere of the Aquatic Macrophyte Littorella uniflora (L.) Ascherson.

Oxygen released by the roots of submerged plants may oxidize organic compounds from the roots and reduced substances continuously supplied by diffusion from the surrounding anoxic hydrosoil. We provide here the first visualization of this gradient environment obtained by microsensor analysis of oxygen in the rhizosphere of the freshwater plant Littorella uniflora (L.) Ascherson. The plants were rooted in an agar medium, in which amorphous FeS provided the main oxygen sink. The oxygen concentration at the root surface ranged from 20 to 450 [mu]M (atmospheric saturation = 280 [mu]M) between darkness and saturating light, and the oxic shell surrounding the roots varied from about 0.5 to 5 mm in thickness. The oxygen flux from the roots was a saturating function of the incident light intensity on the leaves, and the oxygen released was consumed mainly at the fluctuating oxic/anoxic interface. The oxic zones around individual roots are under dynamic control by light, root morphology, root density, and sediment reducing capacity, and, therefore, oxygen concentrations should be subject to substantial diurnal fluctuations in dense Littorella populations in nutrient-poor sediments.

Effects of Sediment Fertility and Initial Plant Density on Growth ofHydrilla verticillata(L.F.) Royle andPotamogeton nodosusPoiret

ABSTRACT In a six-week greenhouse investigation, variations in the growth of Hydrilla verticillata (L.f.) Royle and Potamogeton nodosus Poiret (- P.. americanus) were examined separately under high and low levels of sediment fertility and plant density. For both species, diminished biomass production and shoot lengths accompanied increased root-to-shoot ratios on infertile sediment. In corresponding density/fertility treatments, Potamogeton consistently produced greater total biomass and root-to-shoot ratios than Hydrilla. Shoot production was similar between species on infertile sediment, but on fertile sediment, Potamogeton produced nearly twice as many shoots as Hydrilla. High plant density, combined with high sediment fertility, resulted in a significant reduction in aboveground biomass of individual Hydrilla plants. However, at neither level of sediment fertility did the biomass of individual Potamogeton plants show significant density effects. Growth responses under nutrient-poor sediment conditions...

Factors influencing potential intralake colonization by Myriophyllum spicatum L.

A factorial experimental design was used to determine which of several factors influence Myriophyllum spicatum L. vegetative colonization success. Also, the relationship between total nonstructural carbohydrate (TNC) content, a measure of propagule vigor, and propagule success was detailed. Colonization success in terms of growth and mortality was best during late summer in shallow water on rich organic sediments. Light availability, temperature, and sediment nutrient levels were high during this period. The poorest colonization conditions occurred during early autumn. During this period mortality was highest in deep water in calcareous nutrient poor sediments. Also, naturally produced vegetative fragments grew better than artificially abscised meristems and had higher TNC concentrations. Low TNC levels increased mortality, but they had no effect on subsequent growth, suggesting that naturally produced fragments are more likely to survive the winter than artificial fragments.