Relative effects of nutrient enrichment and grazing on epiphyte-macrophyte (Zostera marina L.) dynamics

The independent and interactive effects of nutrient concentration and epiphyte grazers on epiphyte biomass and macrophyte growth and production were examined in Zostera marina L. (eelgrass) microcosms. Experiments were conducted during early summer, late summer, fall, and spring in a greenhouse on the York River estuary of Chesapeake Bay. Nutrient treatments consisted of ambient or enriched (3× ambient) concentrations of inorganic nitrogen (ammonium nitrate) and phosphate. Grazer treatments consisted of the presence or absence of field densities of isopods, amphipods, and gastropods. epiphyte biomass increased with both grazer removal and nutrient enrichment during summer and spring experiments. The effect of grazers was stronger than that of nutrients. There was little epiphyte response to treatment during the fall, a result possibly of high ambient nutrient concentrations and low grazing pressure. Under low grazer densities of early summer, macrophyte production (g m^–2 d^–1) was reduced by grazer removal and nutrient enrichment independently. Under high grazer densities of late summer, macrophyte production was reduced by enrichment only with grazers absent. During spring and fall there were no macrophyte responses to treatment. The relative influence of epiphytes on macrophyte production may have been related to seasonally changing water temperature and macrophyte requirements for light and inorganic carbon.     

Relations between trophic state indicators and plant biomass in Florida lakes

We collected quantitative data on macrophyte abundance and water quality in 319 mostly shallow, polymictic, Florida lakes to look for relationships between trophic state indicators and the biomasses of plankton algae, periphyton, and macrophytes. The lakes ranged from oligotrophic to hypereutrophic with total algal chlorophylls ranging from 1 to 241 mg m^–3. There were strong positive correlations between planktonic chlorophylls and total phosphorus and total nitrogen, but there were weak inverse relationships between the densities of periphyton and the trophic state indicators total phosphorus, total nitrogen and algal chlorophyll and a positive relationship with Secchi depth. There was no predictable relationship between the abundance of emergent, floating-leaved, and submersed aquatic vegetation and the trophic state indicators. It was only at the highest levels of nutrient concentrations that submersed macrophytes were predictably absent and the lakes were algal dominated. Below these levels, macrophyte abundance could be high or low. The phosphorus–chlorophyll and phosphorus–Secchi depth relationships were not influenced by the amounts of aquatic vegetation present indicating that the role of macrophytes in clearing lakes may be primarily to reduce nutrient concentrations for a given level of loading. Rather than nutrient concentrations controlling macrophyte abundance, it seems that macrophytes acted to modify nutrient concentrations.     

Nitrogen and phosphorus uptake in seedlings of the seagrass Amphibolis antarctica in Western Australia

The uptake of nitrate, ammonium and phosphate was examined in vitro in seedlings of the seagrass Amphibolis antarctica ((Labill.) Sonder ex Aschers.). Uptake of all three nutrients was significantly correlated with external concentration up to 800 µ g l^–1. The uptake of nitrate (0–200 µ g NO₃-N g dry wt^–1 h^–1) was significantly lower than the uptake of ammonium (0–500 µ g NH₄-N g dry wt^–1 h^–1), suggesting that the seedlings have a higher affinity for this form of nitrogen in the water column.Data were in general agreement with uptake rates recorded for other seagrasses, notably Zostera marina. In comparison to the dominant macroalgae for the same region, seedlings had either similar or higher uptake rates in relation to external concentration, lending support to the hypothesis that seedlings, which do not possess roots, behave like macroalgae in terms of nutrient acquisition from the water column.A comparison with literature data on adult seagrass suggests, however, that seagrasses show lower uptake rates than macroalgae suggesting that the macroalgae, which are totally reliant on the water column for nutrients, are more efficient at uptake than seagrasses which may potentially use the sediment for a nutrient source.     

Nutrient dynamics in the floodplain ponds of the Paraná River (Argentina) dominated by the water hyacinth Eichhornia crassipes.

Some aspects of nutrient status and dynamics prevailing during low and high water conditions in the fringing floodplain ponds of the Paraná River dominated by the floating macrophyte Eichhornia crassipes are described. During summertime low water conditions, low DIN:DRP ratios (0.16–1.0) and low DIN (0.5–4.8 mol.liter^–1) in the root-zone of the floating meadows suggest that macrophyte growth is limited by nitrogen. DRP concentrations appear to be controlled more by abiotic sorption-dissolution than by biological reactions. Preflood nutrient fluxes from the sediments, as estimated from porewater profiles, show that a minimum of 1.19 and 0.38 mmol.m^–2.d^–1 of DIN and DRP were regenerated from the sediments, respectively. Heterotrophic N₂ fixation is primarily associated with decaying litter (0.4 to 3.2 molN₂.g^–1.d^–1). Nutrient recycling from sediments and meadow-litter, and heterotrophic N₂ fixation (1.4 mmolN.m^–2.d^–1) appear sufficient to sustain high floating macrophyte productivity for long periods of time, without invoking large inputs from the river. The high water and early isolation periods are characterized by a very dynamic behavior of DIN, reflecting marked imbalances between N supply and demand by the biota. After hydrologic isolation of the ponds, DIN rapidly decreases to undetectable levels and stays low for the following 3 weeks, presumably as a result of high demand by phytoplankton and sediment bacteria. DIN increases again to high values 3–8 weeks after the flood, following the re-establishment of NH₄ ⁺ fluxes from the sediments. Compared to DIN, DRP concentrations remain relatively high and change little during and after the flood. Because of their small amplitude and short duration, floods do not appear to stimulate floating macrophyte production in the Paraná.     

Submerged Macrophyte-bed Effects on Water-Column Phosphorus,Chlorophyll <Emphasis Type="Italic">a</Emphasis>, and Bacterial Production

Submerged macrophytes are a major component of freshwater ecosystems, yet their net effect on water column phosphorus (P), algae, and bacterioplankton is not well understood. A 4-month mass-balance study during the summer quantified the net effect of a large (5.5 ha) undisturbed macrophyte bed on these water-column properties. The bed is located in a slow-flowing (0.05–0.1 cm s^–1) channel between two lakes, allowing for the quantification of inputs and outputs. The P budget for the study period showed that, despite considerable short-term variation, the macrophyte bed was a negligible net sink for P (0.06 mg m^–2 day^–1, range from –0.76 to +0.79 mg m^–2 day^–1), demonstrating that loading and uptake processes in the weedbed roughly balance over the summer. Chlorophyll a was disproportionately retained relative to particulate organic carbon (POC), indicating that the algal component of the POC was preferentially trapped. However, the principal contribution of the weedbed to the open water was a consistent positive influence on bacterioplankton production over the summer. Conservative extrapolations based on measured August specific exports (m^–2 day^–1) of P and bacterial production exiting the weedbed applied to five regional lakes varying in lake morphometry and macrophyte cover suggest that even in the most macrophyte dominated of lakes (66% cover), P loading from submerged weedbeds never exceeds 1% day^–1 of standing epilimnetic P levels, whereas subsidization of bacterioplankton production can reach upward of 20% day^–1. The presence of submerged macrophytes therefore differentially modifies algae and bacteria in the water column, while modestly altering P dynamics over the summer.     

The Utricularia-Cyanophyta association and its nitrogen-fixing capacity

Utricularia inflexa Forsk. growing in a shallow lake near Dar es Salaam, Tanzania was found to be abundantly associated with blue-green algae and other microorganisms, though blue-green algae were not found in the lake water. The epiphytes occurred both on the outer surfaces of the macrophyte and inside its traps. Seven genera of the Cyanophyta were observed, amongst which Anabaena was the most abundantly and consistently associated.Both epiphytes on the outer surfaces of the macrophyte and those inside mature traps fixed nitrogen in situ. Mature parts of the macrophyte fixed nitrogen at higher rates than immature parts, indicating the former was associated with a greater abundance of nitrogen fixers. The Utricularia-Cyanophyta association showed a nitrogen fixation (acetylene reduction) rate of 2012 nmol C₂H₄ g^–1 dry weight h^–1 or, in terms of lake area, of 4 500 nmol C₂H₄ m^–2 h^–1. The relative contributions of blue-green algae and bacteria were not determined.It is proposed that the association may involve a degree of physiological interdependence and that the association has potential as a biofertilizer for rice.Based on an M.Sc. thesis by G. M. Wagner submitted to the Department of Botany, University of Dar es Salaam, 1983.Based on an M.Sc. thesis by G. M. Wagner submitted to the Department of Botany, University of Dar es Salaam, 1983.     

Effect of an aquatic herbicide on sediment nutrient flux in a freshwater marsh

Three concentrations of the herbicide simazine were added to in situ macrophyte-free enclosures with and without sediment contact. Changes in the concentrations of total ammonia, total reactive phosphorus, and silicon were monitored, and net sediment flux was calculated from the difference in nutrient concentration between bottomed and unbottomed enclosures. Rates of sediment release for all three nutrients were unaltered by 0.1 mg · l^–1 simazine in relation to a control, whereas rates were increased proportionally at 1.0 and 5.0 mg · l^–1. These results suggest that increases in dissolved nutrients commonly observed following herbicide treatment of shallow waters may not be attributable solely to macrophyte decay, byt may also involve a complex interaction of biotic and abiotic sediment nutrient exchange processes.Contribution Number 103 from the University of Manitoba Field Station, Delta Marsh, Canada     

Factors controlling phytoplankton primary productivity in Byram Lake,Mt. Kisco,N.Y., summer 1977

Phytoplankton productivity was measured in Byram Lake Reservoir during summer 1977. Depth integrated productivity (0–5 gC m^{– 2} d^–1) increased with station depth, which together with visibility measurements indicated that light did not limit deep station productivity (C1 and S2). Macrophytes at station C5 (shallow) reduced the euphotic zone to 0 in June.On a unit depth basis, C5 was the most productive station. Apparently changes in macrophyte growth, regulated by light and temperature, controlled phytoplankton production. At C1, productivity was related to levels of different nutrients at different depths, the thermocline influencing nutrient availability at mid-depth. At S2, NH₃-N controlled mid-depth productivity. Surface and mid-depth productivity appeared influenced by factors not measured in this study.     

Effect of benzoic acid on the ammonium transport system of Zygosaccharomyces bailii

Summary A study of the ammonium transport system of Zygosaccharomyces bailii was carried out using methylammonium as a non-metabolizable analogue. Benzoic acid in the growth medium decreased the capacity of the transport system from 1.46 ± 0.11 mmol.g^–1.h^–1 to 0.41±0.04 mmol.g^–1.h^–1, while the affinity for ammonium was not significatively affected. Although ammonium uptake was inhibited by benzoic acid, the ammonium transport system was still active at preservative concentrations which fully inhibited growth suggesting that inhibition of growth was not governed by the uptake of this nutrient.     

Physical variables driving epiphytic algal biomass in a dense macrophyte bed of the St. Lawrence River (Quebec, Canada)

The variables affecting epiphyton biomass were examined in a sheltered, multispecies macrophyte bed in the St. Lawrence River. Alteration of light penetration, resulting from the presence of dense macrophytes forming a thick subsurface canopy, primarily determined epiphyton biomass. Seasonal decrease of water levels also coincided with major increases in biomass. Plant morphology was the next important variable influencing epiphytic biomass, whereas the contribution of other variables (sampling depth, macrophyte species, relative abundance of macrophytes, and temperature) was low. Groups of lowest epiphyte biomass (0.1–0.6 mg Chla g^–1 DW) were defined by the combination of a low percentage of incident light (<13% surface light) and simple macrophyte stem types found below the macrophyte canopy. Highest epiphyte biomass (0.7–1.8 mg Chla g^–1 DW) corresponded to samples collected in mid-July and August, under high irradiance (>20% surface light) and supported by ramified stems. Our results suggest that epiphyton sampling should be stratified according to the fraction of surface light intensity, macrophyte architecture, and seasonal water level variations, in decreasing order of influence.     

Using porewater profiles to assess nutrient availability in seagrass-vegetated carbonate sediments

We measured porewater profiles of inorganic (NH₄ ⁺, NO₃ ^–(+NO₂ ^–), PO₄ ^3– (hereafter referred to as DIP)) and organic (DON, DOP) nutrients in seagrass-vegetated sediments at two sites in a shallow bay in Bermuda within close proximity (200 m) but subject to different nutrient loading. At both sites, total dissolved and inorganic nutrient concentrations were usually 1–2 orders of magnitude higher in the sediments than in the water column, with the exception of NO₃ ^–. Organic N and P were significant components of the total dissolved nutrient pools both in the sediment porewater and in the overlying water column (up to 75% for DON and 40% for DOP), and may be important in meeting plant nutrient demands. We used two approaches to examine how well porewater nutrient concentrations reflected the relative availabilities of N and P for seagrasses: (1) a simple stoichiometric nutrient regeneration model based on the N:P ratio of decomposing organic matter and porewater NH₄ ⁺ concentrations to predict porewater DIP, and (2) fitting of the porewater profiles to estimate rates of net nutrient production (or consumption), which reflects the balance between nutrient sources and sinks in the rhizosphere. The stoichiometric model indicated that sediment porewaters were depleted in P relative to N in the low-nutrient outer bay site, and enriched in P relative to N in the higher-nutrient inner bay site. These results are consistent with the mechanism of carbonate sediments in oligotrophic tropical environments being a strong sink for dissolved inorganic P and our previous work suggesting that nutrient enrichment causes P to become disproportionately more available than N. Net nutrient production rates of porewater P at both sites and N at the inner bay site were low (typically < 2%) relative to the nutrient demands of the seagrasses. The implications of the profile interpretation are two-fold: (1) the low rates of net nutrient production indicate diffusive losses from the root zone were insignificant and that nutrient turnover rates were high, except in the P-limited outer bay where N accumulated in sediment porewaters; and (2) because standing stock nutrient concentrations often represent a small fraction of the total nutrients cycled in the sediments, they are in many cases a poor indicator of nutrient availability. Based on our estimates of losses from the root zone, decomposition, and plant uptake we have constructed a rough budget for the cycling of P and N at our two sites.     

Benthic oxygen respiration, ammonium and phosphorus regeneration in surficial sediments of the Sacca di Goro (Northern Italy) and two French coastal lagoons: a comparative study

During 1994 net sediment-water fluxes of oxygen, ammonium and inorganic phosphorus as well as sediment profiles of organic matter, nitrogen, phosphorus and iron were determined in three shallow eutrophic environments. Investigations were conducted monthly from March to December at five stations in the Sacca di Goro (Po River Delta, Italy). In the late summer, samples were collected from a single site in the Prévost lagoon (French Mediterranean coast) and three stations in the Bassin d'Arcachon (French Atlantic coast). In the Sacca di Goro, water-sediment exchanges of O₂, NH ₄ ⁺ and PO ₄ ^3– were estimated by means of core incubation in the dark. Benthic fluxes for the French lagoons were in part determined experimentally using benthic chambers and in part from the literature.In general in the Sacca di Goro the highest oxygen uptake and nutrient release rates were found at the central sites, affected by macroalgal growth. At the sampling site adjacent to the freshwater inlet, sediment-water exchanges were principally influenced by tidal activity. In terms of organic matter and nutrient levels, sediments from the Sacca di Goro and from the Prévost lagoon, both colonised by the floating macroalga Ulva rigida C. Agardh, were similar. Sediments from the inner sheltered site in the Bassin d'Arcachon, invaded by the rooted macrophyte Ruppia cirrhosa (Pet.) Grande, showed the highest total N and P content (363 ± 157 µmol N cm^–3 and 15 ± 2 µmol P cm^–3 as average values in the top 10 cm of sediment), but were low in pore water ammonium and orthophosphate probably due to the high sequestering capacity of the system and/or efficient coupling between bacterial nutrient regeneration and assimilation by the plant roots. In addition the outer tidal stations in the Bassin d'Arcachon, invaded by rooted macrophytes, were low in pore water nutrients. A different trend was evident in the Prévost lagoon where the concentrations of exchangeable inorganic phosphorus and ammonium were appreciable (0.28 ± 0.07 µmol P cm^–3 and 2.4 ± 1.4 µmol N cm^–3 as average values in the top 10 cm of sediment). High amounts of dissolved organic nitrogen were found in the pore water at all the sites investigated showing the key role of the organic nitrogen in the recycling of nitrogen in these systems.The hypothesis that iron is a key factor in controlling phosphorus release is discussed since the Sacca di Goro, which is subject to dystrophic crises, is richer in iron than the Bassin d'Arcachon, which is a more buffered system.     

Vernal nitrogen and phosphorus retention by forest understory vegetation and soil microbes

Northern hardwood forests experience annual maximal loss of nutrients during spring. The vernal dam hypothesis predicts that spring ephemeral herbs in northern hardwood forests serve as sinks for nutrients during this season and reduce the loss of nutrients from the terrestrial ecosystem. Soil microbes of northern hardwood forests also sequester nutrients during spring. We compared the vernal nutrient acquisition ability of a soil microbial community and an understory plant community with species of mixed leaf phenology. We monitored nitrogen and phosphorus pool sizes in understory vegetation and soil microbes during spring from 1999 through 2001 in a northern hardwood forest in the Catskill Mountains, New York. Vegetation nutrient content increased during two spring seasons by an average of 3.07 g N m^–2 and 0.19 g P m^–2 and decreased during one spring by 0.81 g N m^–2 and 0.10 g P m^–2. Evergreen, wintergreen, and deciduous plant species were able to sequester nutrients during spring. Soil microbial nutrient content decreased during one spring by 1.29 g N m^–2 and remained constant during the other two springs. Streamwater nitrogen losses were not correlated with biotic nutrient uptake suggesting a temporal disconnect between the two processes. We conclude that understory vegetation is a larger potential sink for vernal nutrients than are soil microbes in this northern hardwood forest and understory and species representing multiple phenologies are capable of vernal nutrient uptake.     

Nitrogen and base cation uptake in seedlings of Acer pseudoplatanus and Calamagrostis villosa exposed to an acidified environment

The effects of solution acidity and form of nitrogen on net nutrient uptake rates in Acer pseudoplatanus and Calamagrostis villosa seedlings were examined as part of a complex ecological study. Uptake rates were measured by the depletion method under controlled conditions (temperature 20 °C, irradiance 400 mol m^–2 s^–1 PAR) from a nutrient solution containing 1.5 mM nitrogen in the form of nitrate or ammonium or an equimolar mixture of both. The solution acidity was kept constant at pH 5.5 (control treatment), 4.5 or 3.5 (low pH treatments). Strongly acid pH decreased or stopped the uptake rates of NO₃ ^–, Mg²⁺ and Ca²⁺, but the uptake of NH₄ ⁺ was not changed in both species. Ammonium ions reduced the uptake rate of NO₃ ^– in Acer but increased the uptake rate in Calamagrostis. Ammonium as the sole source of nitrogen had a strong negative impact on the uptake rates of calcium and magnesium and this effect was independent of the media acidification usually connected with NH₄ ⁺ uptake and assimilation. However, the negative effect of ammonium ions on the base cation uptake was more pronounced at low pH values.     

Phytoplankton primary production and nutrients in the Oosterschelde (The Netherlands) during the pre-barrier period 1980–1984

Phytoplankton primary production, nutrient concentrations and turbidity were monitored at three stations in the Oosterschelde during 1980–1984 as part of an ecosystem study.From comparisons of dissolved nutrient ratios with the nutrient requirements of phytoplankton, and of ambient nutrient concentrations with half-saturation constants for nutrient uptake by natural phytoplankton populations it was concluded that silicate was a limiting nutrient for diatoms after the spring bloom until the end of the summer. Dissolved inorganic nitrogen and phosphate were not considered to be limiting to phytoplankton growth.In general, the phytoplankton growing season started during the first fortnight of April and ended at the end of September. Column production in the whole Oosterschelde varied between 201 and 540 g C m^–2 yr^–1 and was, on average, 25% higher in the western part than in the eastern part. Basin production in the Oosterschelde varied between 120 and 466 g C m^–2 yr^–1 and was, on average, 55% higher in the western part than in the eastern part; this difference could be explained by differences in the ratio of euphotic depth to mean depth of the compartments.Estimated carbon-specific growth rates in the eastern part varied between < 0.1 and 3 d^–1 and in the western part between < 0.1 and 1 d^–1. This difference could be explained by the great differences in depth of the compartments. Carbon-specific growth rates are discussed in relation to phytoplankton loss rates. It is suggested that in the eastern part sedimentation must be an important sink for phytoplankton.Communication no. 473 of the Delta Institute for Hydrobiological Research, Yerseke, The Netherlands.     

Competition in tree row agroforestry systems. 2. Distribution, dynamics and uptake of soil inorganic N

The effect of tree row species on the distribution of soil inorganic N and the biomass growth and N uptake of trees and crops was investigated beneath a Grevillea robustaA. Cunn. ex R. Br. (grevillea) tree row and Senna spectabilisDC. (senna) hedgerow grown with Zea mays L. (maize) and a sole maize crop, during one cropping season. The hypothesis was that a tree with a large nutrient uptake would have a greater competitive effect upon coexisting plants than a tree that takes up less and internally cycles nutrients. The field study was conducted on a kaolinitic Oxisol in the sub-humid highlands of western Kenya. Soil nitrate and ammonium were measured to 300 cm depth and 525 cm distance from the tree rows, before and after maize cropping. Ammonium concentrations were small and did not change significantly during the cropping season. There was > 8 mg nitrate kg^–1 in the upper 60 cm and at 90–180 cm depth at the start of the season, except within 300 cm of the senna hedgerow where concentrations were smaller. During the season, nitrate in the grevillea-maize system only decreased in the upper 60 cm, whereas nitrate decreased at almost every depth and distance from the senna hedgerow. Inorganic N (nitrate plus ammonium) decreased by 94 kg ha^–1 in the senna-maize system and 33 kg ha^–1 in the grevillea-maize system.The aboveground N content of the trees increased by 23 kg ha^–1 for grevillea and 39 kg ha^–1 for senna. Nitrogen uptake by maize was 85 kg ha^–1 when grown with grevillea and 65 kg ha^–1 with senna. Assuming a mineralisation input of 50 kg N ha^–1season^–1, the decrease in inorganic soil N approximately equalled plant N uptake in the grevillea-maize system, but exceeded that in the senna-maize system. Pruning and litter fall removed about 14 kg N ha^–1 a^–1 from grevillea, and > 75 kg N ha^–1 a^–1 from senna. The removal of pruned material from an agroforestry system may lead to nutrient mining and a decline in productivity.     

Growth analysis of soil-grown spinach plants at different N-regimes

Spinach plants were grown in pots under controlled conditions in three different soils (a loamy sand, a silt loam at low mineral-N level and a silt loam at the double mineral-N level). The nitrogen uptake pattern varied considerably between the three soil types and was used to validate an equation between the relative growth rate and nitrogen content. This equation is based on the growth response of spinach plants grown hydroponically at equal environmental conditions either at optimum nitrogen supply (complete nutrient solution) or with a relative nitrate addition rate of 0.30 day^–1, 0.225 day^–1 or 0.15 day^–1 effecting an exponential increase in nitrogen uptake. Growth in potted soil was slightly overestimated. Part of this bias was explained by the lower shoot weight ratio observed for the soil grown plants. This was demonstrated by the improvement in growth predictions when using net assimilation rate rather than relative growth rate as the driving variable in the model.     

Chlorophyll content of seston in a regulated Rocky Mountain river,Idaho, USA

The chlorophyll content of seston at four sites in a regulated mid-order Rocky Mountain river, Henry's Fork of the Snake River, Idaho, USA, was examined. Spectrometry was used to determine the amount of chlorophyll a, b, c and in seston and potential organic matter sources including macrophytes, algae, plankton, and terrestial plant litter. The amount of chlorophyll in seston varied among size fractions, sites, and seasons. Coarse seston (6–1 mm) contained the most chlorophyll a (2.3 mg g^–1 ashfree dry mass [AFDM]) followed by ultra fine seston (53–0.3 µm; 1.9 mg g^–1 AFDM), very coarse seston (> 6 mm, 1.8 mg g^–1 AFDM), very fine seston (250–53 µm; 1.3 mg g^–1 AFDM), and fine seston (1–0.25 mm; 0.7 mg g^–1 AFDM). Chlorophyll content of coarse seston was similar at all sites reflecting a common source, aquatic macrophyte debris. Chlorophyll content of coarse and fine seston were highest in fall reflecting the importance of phenology of aquatic plants on sestonic pigment levels. Very fine seston from below a reservoir contained more chlorophyll than seston from downstream and tributary sites suggesting a reservoir source. Terrestial plant litter was chlorophyll depleted compared to autochthonous materials and seston. Most seston was autochthonously-derived from a variety of macrophyte, algal, and planktonic sources. The chlorophyll content of Henry's Fork seston was higher than that reported for similar rivers. Island Park Dam moderates river temperature and flow, enhances autotrophy, and accounts for the consistently high sestonic chlorophyll levels.     

Outdoor pond cultivation of the subtropical marine red algaGracilaria tenuistipitata in brackish water in Sweden. Growth,nutrient uptake,co-cultivation with rainbow trout and epiphyte control

Pond cultivation of the subtropical, euryhaline macroscopic red algaGracilaria tenuisipitata var.liui Zhanget Xia was carried out in brackish seawater (6–7) in the Gryt archipelago on the east coast of Sweden, using four outdoor tanks of 30–40 m³. Growth rate and nutrient uptake in batch culture were measured with the aim of estimating the water purification capacity ofG. tenuisipitata in outdoor conditions. Its ability to withstand epiphytic infections was also studied. An average growth rate of 4 biomass increase per day was recorded during two seasons with a maximum growth rate of 9 d^–1. The initial biomass was usually 1 kgFW m^–3 (FW, fresh weight). The nutrient uptake capacity was on average ca. 1 g N_i kgFW^–1 d^–1 and 0.08 g P_i kgFW^–1 d^–1 and the uptake rates for NH₄ ⁺-N were higher than those for NO₃ ^–-N. Both the growth rate and the nutrient uptake rate were highest at the highest water temperature. Co-cultivation with rainbow trout (Oncorhynchus mykiss) was tested: with trout fodder as the only nutrient inputG. tenuistipitata could grow and maintain low levels of N_i and P_i with optimum efficiency at a trout: alga ratio of 1:1 (w:w). Epiphytic growth of filamentous green and brown algae was limited, probably as a result of the high pH values caused by inorganic carbon uptake byG. tenuistipitata. The growth ofEnteromorpha intestinalis, the only significant epiphyte, was completely inhibited and the majority of plants died by a few days treatment with 100 µg 1^–1 Cu²⁺, a concentration that did not severely affectG. tenuistipitata. We conclude thatG. tenuistipitata can be cultivated in outdoor ponds in southern Sweden during 5–6 months of the year using aerated or unaerated batch cultures and that wastewater from trout cultivation may be used as a nutrient source, resulting in purification with respect to N and P.     

Heterotrophic bacterial activity in Roskilde Fjord sediment during an autumn sedimentation peak

Total oxygen uptake, bacterial oxygen uptake, total bacterial biomass and active bacterial biomass were determined at the sediment-water interface at two stations in the brackish Roskilde Fjord between September and December in 1986 before, during and after sedimentation of a phytoplankton bloom. Bacterial oxygen consumption was separated from total oxygen consumption by addition of cycloheximide. The fractional and the absolute bacterial oxygen uptake were greatest at the most eutrophic station, where total oxygen uptake was 870–1740 mg O₂ m^–2 d^–1 and the bacterial oxygen uptake was 232–870 mg O₂ m^–2 d^–1. At the less eutrophic station, total oxygen uptake was 725–1740 mg O₂ m^–2 d^–1. and bacterial oxygen uptake was 200–550 mg O₂ m^–2 d^–1.Active bacterial biomass was separated from total bacterial biomass by addition of the terminal electron acceptor INT-formazan. The active bacterial biomass was 70–120 µg C mg^–1 ww of sediment at the most eutrophic station and 50–90 µg C g^–1 ww of sediment at the other station. Differences in capacity of bacterial oxygen uptake between the two stations correlated to the active bacterial biomass. The non-temperature dependent bacterial oxygen uptake correlated with the sedimentation rate.