Geochemical composition,phosphorus speciation and mass transport of fine-grained sediment in two Lake Erie tributaries

The concentration of major elements (Si, Al, Ca, Mg, Na, K, Fe, Ti, Mn and P), particulate phosphorus forms (NH₄Cl-RP, BD-RP, NaOH-RP, HCl-RP and NaOH₍₈₅₎-RP) and carbon content were determined in six size fractions (<8, 8–12, 12–19, 19–31, 31–42 and 42–<60 µm) of sediment collected at gauging stations located in two Lake Erie tributaries (Big Creek and Big Otter Creek). Concentrations of major elements and phosphorus forms were remarkably similar in sediment size fractions from both rivers. Nonapatite inorganic P (NAIP) and organic P (OP) concentrations increased with decreasing grain size while apatite inorganic P (AIP) content decreased with decreasing grain size. Results of phosphorus fractionation studies were combined with historical (particle size) and hydrometric data to simulate the export of particle P on tributary sediment < 63 µm. AIP represents 67 and 70% of the calculated particulate P mass while NAIP accounts for 26 and 23% of sediment-bound P transported in Big Otter Creek and Big Creek, respectively. The < 8 µm size fraction of tributary sediment is the most significant for the potential release of bioavailable P into the water column.     

An experimental study of the effects of fish zooplanktivory on the phytoplankton of a Brazilian reservoir

Two short-term (4–5 week) sets of enclosure experiments were conducted during winter periods (1982, 1983) in Americana Reservoir near Sao Paulo to test effects of fish predation reducing grazing pressure by large cladoceran zooplankton (mainlyDaphnia gessneri) on phytoplankton density, cell size distribution, biomass, species composition and richness. Two enclosures were stocked with the zooplanktivorous characin fishAstyanax whereas two others remained fishless during each set of experiments. Within two weeks or less, phytoplankton cell density and biomass were significantly reduced in the fishless enclosures, particularly in the small cell size (< 15 µm) fraction. Changes also occurred in species composition and richness. These shifts generally were maintained or intensified during the course of the experiments.     

Whole-stream phosphorus release studies: variation in uptake length with initial phosphorus concentration

A new dual channel flow injection analyser that can simultaneously analyse soluble reactive phosphorus and bromide in the field, has been used in an experiment to test the hypothesis that the phosphorus uptake length in Myrtle Creek, a small forested stream in the Australian Highlands, is influenced by the initial phosphorus concentration used in whole-stream release studies. The phosphorus uptake length was found to decrease with decreasing initial phosphorus concentration added; the uptake length was 98 m when an initial P concentration of 51.0 µg 1^–1 was used, 90 m with 21.7 µg 1^–1 and 63 m with 12.7 µg 1^–1. The estimated errors in the uptake lengths were 6–8%. Approximately 32% of the added phosphorus was retained in the 32 m study reach, with almost all (ca. 93%) of this retained phosphorus taken up by the sediments (microbial uptake plus physico-chemical adsorption) and only a small amount retained in transient storage zones.     

Potential Importance of Fish Predation and Zooplankton Grazing on Natural Populations of Freshwater Bacteria

The rates of ingestion of natural bacterial assemblages by natural populations of zooplankton (>50 μm in size) were measured during a 19-day period in eutrophic Frederiksborg Slotssø, Denmark, as well as in experimental enclosures (containing 5.3 m³ of lake water). The fish and nutrients of the enclosures were manipulated. In enclosures without fish, large increases in ingestion by zooplankton >140 μm in size were found (up to 3 μg of C liter⁻¹ h⁻¹), compared with values less than 0.3 μg of C liter⁻¹ h⁻¹ in the enclosures with fish and in the open lake. Daphnia cucullata and D. galeata dominated the community of zooplankton of >140 μm. Ingestion rates for zooplankton between 50 and 140 μm decreased after a period of about 8 days, in all enclosures and in the lake, to values below 0.1 μg of C liter⁻¹ h⁻¹. On the last 2 sampling days, somewhat higher values were observed in the enclosures with fish present. The >50-μm zooplankton ingested 48 to 51% of the bacterial net secondary production in enclosures without fish, compared to 4% in the enclosures with added fish. Considering the sum of bacterial secondary production plus biomass change, 35 to 41% of the available bacteria were ingested by zooplankton of >50 μm in the enclosures without fish, compared with 4 to 6% in the enclosures with added fish and 21% in the open lake. Fish predation reduced the occurrence of zookplankton sized >50 μm and thus left a large proportion of the available bacteria to zooplankton sized <50 μm. In fact, there were 4.6 × 10³ to 5.0 × 10³ flagellates (4 to 8 μm in size) ml⁻¹ in the enclosures with fish added as well as in the lake, compared with 0.5 × 10² to 2.3 × 10² ml⁻¹ in the enclosures without fish. This link in the food chain was reduced when fish predation on zooplankton was eliminated and a direct route of dissolved organic matter, via the bacteria to the zooplankton, was established.     

Biomass and photosynthesis of size-fractionated phytoplankton in Canadian Shield lakes

Both in situ primary production and biomass (chlorophyll ) of fractionated phytoplankton (<64,µ, <25 µm and < 10 µm) were studied in 10 Canadian Shield lakes to elucidate the spatial and temporal variability of the contribution of size fractions to the biomass and primary production of the phytoplankton community. Mean summer biomass and production of each size fraction varied significantly between lakes. Within lakes, temporal variation was low for biomass but great for production. However, temporal variation can be considered of minor importance during the sampling period, as compared to the spatial variation between lakes. Algae from the < 10 µm size fraction were the most important in biomass (41–65 %) and production (23–69%). The temporal trends for both phytoplankton variables thus generally followed closely that of the < 10 µm size fraction. Among the physical, chemical and morphometric variables of the studied lakes, water transparency (Secchi disk), total phosphorus, lake volume, lake area, and mean depth gave the best correlations with phytoplankton variables.Contribution number 354 from the Groupe de recherches en Ecologie des Eaux douces, Limnological Research Group, Université de Montréal.     

Consumer-dependent responses of lake ecosystems to nutrient loading

The nutrient loading concept proposes that algal biomass, waterclarity and the processes of lake eutrophication are a functionof nutrient loading. We hypothesized that grazers play an importantrole in determining the impacts of nutrient loading on algalbiomass and water clarity, and the overall eutrophication process.To test how the contrasting grazer communities modify the fateof nutrients, we added nutrients (nitrate and phosphate) ata known loading rate to four large enclosures, but in two ofthe four enclosures large cladoceran grazers (Daphnia >1mm mean length) were allowed to develop by removing the planktivorousfish. In the remaining two enclosures, the development of largeDaphnia was prevented by adding planktivorous fish. The concentrationsof epilimnetic total phosphorus (TP) increased at a similarrate in all four enclosures. However, the daily accumulationof added phosphate into the participate or planktonic forms,especially into plankton <20 µm, was three times fasterwhen large Daphnia were absent than when large Daphnia wereabundant. In the enclosures with large Daphnia, added phosphatewas accumulated in the dissolved pool instead. At a constantnutrient loading, algal biomass (chlorophyll a) increased fourtimes faster in the enclosures without large Daphnia than inthose with large Daphnia. Similarly, Secchi depth declined from3.5 to <1 m when Daphnia were absent, but did not declinewhen Daphnia were common. Our results demonstrate that the samenutrient loading and the resultant increase in epilimnetic TPdo not produce the same trophic conditions, as indicated byalgal biomass and water clarity, if the grazers of the majorassimilators of nutrients (the fraction of plankton edible toDaphnia) are different. We suggest that stratified lake ecosystemshaving functionally dominant large grazer communities may beless prone to eutrophication than those lacking large grazers.Consistent with the nutrient loading concept, epilimnetic concentrationsof phosphorus increase proportionately with increased loadingof phosphorus, but the trophic conditions of ecosystems indicatedby algal biomass and water clarity do not follow the same patternsunder contrasting conditions of grazer communities. We suggestthat models predicting algal biomass from loading rates shouldaccount for the role of grazers.     

Phosphorus recycling in lakes: evidence from large limnetic enclosures for the importance of shallow sediments

• 1 The dynamics of phosphorus in large field enclosures are described from a series of experimental manipulations directed primarily to the study of phytoplankton populations. The enclosures were similar in size but differed in their depth contours. The aim of the present analysis was to detect systematic differences in the phosphorus budgets of deep and shallow systems.
• 2 The sequence of events in each of six year-long experiments is described in terms of algal production. The values are compared with water-column measurements of total phosphorus (TP), the reactive fraction (SRP) and changes in the particulate fraction (PP, assumed to be TP minus SRP). Stoichiometric ratios for larger phytoplankton crops were in the range 85–110 C/P (molar).
• 3 The annual aggregate of new carbon produced in the two uniformly deep enclosures (11.5 m) in relation to the phosphorus supplied was also remarkably close to Redfield stoichiometry, with between 87 and 97mol C yielded per mol P added. However, in the graded enclosure (< 4–13.5 m), the yield was up to twice as great (maximum: 197 mol C mol^-1 P available).
• 4 There are two possible mechanisms. A graded surface permits a greater area of sediment to be in contact with the hypolimnion and for the sediment oxygen demand to deplete a greater hypolimnetic volume; the associated fall in redox would have been adequate to explain the observed increment of dissolved phosphorus to the hypolimnion, associated with the Einsele–Mortimer model (ferric → ferrous iron reduction). There was no clear evidence that this phosphorus supported autotrophic production until late in the year.
• 5 The alternative mechanism is based on the association of phosphorus ‘injection’ to the epilimnion during wind-mixed episodes, when the enhanced shear stress on shallow sediments resuspends fine, superficial deposits and their interstitial waters into the full circulation. Circumstantial evidence suggests this second pathway predominated in the graded enclosure but was ineffective in the uniformly deep enclosures.
• 6 The role of mechanical, aerobic phosphorus cycling in the nutrient economy of lakes, especially small or shallow ones, may have been underestimated in the past but it must be considered when evaluating any proposal to reduce the external nutrient loads to lakes.

     

Scaling food chains in aquatic mesocosms: do the effects of depth override the effects of planktivory?

To assess the effects of physical dimension and planktivorous fish on phytoplankton standing crop, we repeated an experiment at different scales in plastic enclosures during summer 1995 in Lake Créteil, France. Enclosures were scaled for a constant surface (1.5 × 1.5 m) as depth was increased from 2.5 to 4.5 m. Even-link (zooplankton and phytoplankton) and odd-link (planktivorous fish, zooplankton and phytoplankton) food webs were established in both shallow and deep enclosures. Fish densities in the deep enclosures were scaled to allow comparisons with shallow ones for both in individuals m⁻² or individuals m⁻³. We explicitly designed this experiment to examine the scale-dependent behavior of the top-down mechanism of algal biomass control in lakes, and in particular to test the hypothesis of stronger cascading effects of fish on lower trophic levels at reduced depth. Both fish and enclosure size had highly significant effects on phytoplankton biomass over the duration of the experiment. No depth × fish interaction effects were observed. The presence of planktivorous fish enhanced phytoplankton biomass in both shallow and deep enclosures, although the reduction in depth generally produced a stronger effect. The mean concentration of chlorophyll a in the deep odd-link systems (ca 5 mg m⁻³) was lower than in the shallow even-link systems (ca 17 mg m⁻³). Statistical interpretation did not change when data were expressed as phytoplankton biomass per unit of surface area. Light limitation and zooplankton grazing are the most probable mechanisms explaining our results in these nutrient-enriched systems. Moreover, we found that the strength of the cascading effect of fish on plankton was not a function of depth. We believe that further studies on scaling effects should be conducted in order to improve our understanding of ecological patterns and to extrapolate results from micro/mesocosms to natural ecosystems. Received: 18 January 1999 / Accepted: 7 June 1999     

Effects of nutrients and grazers on periphyton phosphorus in lake enclosures

SUMMARY. 1. Periphyton. measured as particulate phosphorus (PP) and expressed as periphyton PP, growing on vertically oriented substrata (polyvinyl impregnated nylon) under different nutrient loadings, light intensities (exposures), and grazer communities was examined in eight large enclosures (750 m3) where nutrients (N and P) and planktivorous fish (1+yellow perch) were added in a 2x2 factorial design.
2. During the first 3 weeks of the experiment (25 June to 15 July), there was a significantly higher accumulation of phosphorus into periphyton (periphyton PP) with fertilization, but fish addition had no effect. During the fourth to seventh weeks (16 July to 12 August), addition of fish was associated with lower abundance of amphipods and chironomids and higher concentration of periphyton PP. In the enclosures without fish, these invertebrates were over 25 times more abundant, and periphyton PP decreased substantially compared to the June-July period. Fertilization increased periphyton PP only at high exposures in the enclosures with fish.
3. Exposure had a significant effect on periphyton PP. In the enclosures with fish, high abundance of nanoplankton reduced water transparency, and periphyton PP was lower in the deeper waters which may have been due to limitation by low light. Lower periphyton PP was also observed at the surface on sunny sides of enclosures without fish, and therefore with high water transparency. This pattern may have been due to inhibitory effects of high light intensity.
4. Periphyton communities in the enclosures with fish had higher uptake rates for planktonic phosphorus, and lower rates of phosphorus release, suggesting that periphyton with high phosphorus demand may have high internal cycling of assimilated phosphorus.     

Mesh-size effects on drift sample composition as determined with a triple net sampler

Nested nets of three different mesh apertures were used to study mesh-size effects on drift collected in a small mountain stream. The innermost, middle, and outermost nets had, respectively, 425 µm, 209 µm and 106 µm openings, a design that reduced clogging while partitioning collections into three size groups. The open area of mesh in each net, from largest to smallest mesh opening, was 3.7, 5.7 and 8.0 times the area of the net mouth. Volumes of filtered water were determined with a flowmeter. The results are expressed as (1) drift retained by each net, (2) drift that would have been collected by a single net of given mesh size, and (3) the percentage of total drift (the sum of the catches from all three nets) that passed through the 425 µm and 209 µm nets. During a two day period in August 1986, Chironomidae larvae were dominant numerically in all 209 µm and 106 µm samples and midday 425 µm samples. Large drifters (Ephemerellidae) occurred only in 425 µm or 209 µm nets, but the general pattern was an increase in abundance and number of taxa with decreasing mesh size. Relatively more individuals occurred in the larger mesh nets at night than during the day. The two larger mesh sizes retained 70% of the total sediment/detritus in the drift collections, and this decreased the rate of clogging of the 106 µm net. If an objective of a sampling program is to compare drift density or drift rate between areas or sampling dates, the same mesh size should be used for all sample collection and processing. The mesh aperture used for drift collection should retain all species and life stages of significance in a study. The nested net design enables an investigator to test the adequacy of drift samples.     

The role of Gloeotrichia echinulata in the transfer of phosphorus from sediments to water in Lake Erken

The abundance of Gloeotrichia echinulata colonies in the sediments of Lake Erken and their phosphorus content were investigated to determine the contribution of Gloeotrichia colonies to total sediment phosphorus. Moreover, the potential size of the algal inoculum and the migration to the water during summer were estimated.The surplus phosphorus content of the resting colonies in the sediment was about 45% of total phosphorus, which maximized at 8.5 µg P (mg dw)^–1 or 81 ng P colony^–1. The C:P ratio (by weight) in the early colonies appearing in the lake water was 50:1, while the ratio stabilized at 150 during the major migration period. The internal supply of surplus phosphorus was used during the pelagic growth of the colonies.The internal phosphorus loading to the epilimnion of Lake Erken due to Gloeotrichia migration could, from the measurements of the increase in particulate epilimnetic phosphorus, be estimated at 40 mg P m ^–2 or 2.5 mg P m^–2 d^–1 in late July and early August. Determination of the number of colonies in the sediment before and during the migration verified this value to be a conservative estimate of the internal phosphorus loading due to Gloeotrichia migration to the epilimnion in Lake Erken.The sediment P content calculated from the P concentration in early epilimnion colonies resulted in a value of 35 µg P (g dw)^–1 as a maximum. This corresponds to only 3% of the total phosphorus content in Lake Erken sediment.     

Intrinsic and extrinsic variables controlling the productivity of asexual populations of Nais spp, (Naididae,Oligochaeta)

The productivity of Nais spp. from periphyton of fishponds of the Dombes area (Ain) was studied in semi-natural conditions by cultivation of zooids in experimental glass enclosures immersed in situ and filled with pond water receiving injections of fertilizers (P₂O₅) and natural filtered periphyton extracts (particles < 70 µm). The growth rate of the experimental populations was not significantly affected by the concentration of fertilizers added to culture media. On the contrary, the water management of the culture media (as renewal or non-renewal of the water in experimental enclosures), the closing procedure of the enclosures and the load and composition of the nutritive substrate controlled the produced biomass. Temperature and food supply were the principal extrinsic variables controlling the asexual growth rate of the Nais species. The stolonization rate was analyzed as a biological parameter implicated in the instantaneous birth rate of zooids and the growth of naidid populations.     

Chlorophyll-nutrient relationships in North Island lakes (New Zealand)

A model for the prediction of chlorophyll a in the near surface waters (1 m) of North Island lakes was developed using data from the literature and our own study of 12 North Island lakes. Annual geometric mean concentrations of chlorophyll a, total nitrogen and total phosphorus were used since no distinct growing season was discernable. Annual mean ratios of total nitrogen to total phosphorus in the near surface waters ranged between 10 and 59 (by weight). Strong correlations were obtained between log-transformed values of chlorophyll a and total nitrogen (r² = 0.53, n = 16), chlorophyll a and total phosphorus (r² = 0.71, n = 21), and between total nitrogen and total phosphorus (r² = 0.69, n = 16). However, after correcting for the high interdependency between total nitrogen and total phosphorus, only total phosphorus was found to be important in predicting chlorophyll concentrations. Much of the variance in the chlorophyll-phosphorus relation was attributable to differences in mean lake depth. Lakes with mean depths less than 11 m had significantly more chlorophyll a per unit of total phosphorus ( = 0.54 µg · µg^–1, SE = 0.05, n = 6) than lakes of greater mean depth ( = 0.17 µg · µg^–1, SE = 0.02, n = 14). When the effect of mean depth was taken into account, 89% of the variance in chlorophyll a was explained compared with 71% for the simple linear regression on total phosphorus alone.     

利用大型围隔研究沉水植被对水体富营养化的影响

在严重富营养化的东湖设置大型围隔（20×29m）两个,并在其中引种沉水植物—菹草（PotamogetoncrispusL．）,探讨了菹草的恢复对水体富营养化的影响。菹草的恢复,使两个大型围隔中的各种营养盐水平都显著地低于围隔外围的湖水（P＜0．01）,溶解氧和透明度显著升高,电导率明显下降,即水质得到了明显的改善。又将其中一个围隔中已恢复的菹草全部收割并移出,在一个多月的期间,未被收割的围隔内水体中各种营养盐浓度比已收割围隔稍低,但未能达到统计学上的显著差异（P＞0．6）,对水体浮游植物叶绿素a浓度也没有显著影响。即在短期（一个月左右）未使水质明显恶化。     

Community development and modes of phosphorus utilization in a late summer ecosystem in the central Gulf of Finland, the Baltic Sea

The development of a filamentous, nitrogen-fixing cyanobacterial bloom was followed during July–August 1990 in a stratified basin in the central Gulf of Finland, Baltic Sea. Hydrography, dissolved inorganic, particulate and total nutrients, chlorophyll a, alkaline phosphatase activity, ³²PO₄-uptake and phytoplankton species were measured. The study period was characterized by wind-induced mixing events, followed by marked nutrient pulses and plankton community responses. Phosphate uptake was highest throughout the study period in the size fraction dominated by bacteria and picocyanobacteria (< 2 µm) and the proportion of uptake in the size fraction 2–10 µm remained low (2–6%). Higher phosphate turnover times were observed in a community showing signs of enhanced heterotrophic activity. The bloom of filamentous, nitrogen-fixing cyanobacteria Aphanizomenon flos-aquae was promoted by a nutrient pulse with an inorganic nutrient ratio (DIN:DIP) of 15. The results show that the quality, frequency and magnitude of the physically forced nutrient pulses have an important role in determining the relative share of the different modes of phosphorus utilization and hence in determining the cyanobacterial bloom intensity and species composition in the Baltic Sea.     

Effects of experimentally induced cyanobacterial blooms on crustacean zooplankton communities

SUMMARY 1. Large in situ enclosures were used to study the effects of experimentally induced cyanobacterial blooms on zooplankton communities. A combination of N and P was added to shallow (2 m) and deep enclosures (5 m) with the goal of reducing the TN : TP ratio to a low level (∼5 : 1) to promote cyanobacterial growth. After nutrient additions, high biomass of cyanobacteria developed rapidly in shallow enclosures reaching levels only observed during bloom events in eutrophic lakes.
2. In the shallow enclosures, particulate phosphorus (PP) was on average 35% higher in comparison with deep enclosures, suggesting that depth plays a key role in P uptake by algae. Phytoplankton communities in both deep and shallow enclosures were dominated by three cyanobacteria species – Aphanizomenon flos-aquae , Anabaena flos-aquae and Microcystis aeruginosa – which accounted for up to 70% of total phytoplankton biomass. However, the absolute biomass of the three species was much higher in shallow enclosures, especially Aphanizomenon flos-aquae . The three cyanobacteria species responded in contrasting ways to nutrient manipulation because of their different physiology.
3. Standardised concentrations of the hepatotoxic microcystin-LR increased as a result of nutrient manipulations by a factor of four in the treated enclosures. Increased biomass of inedible and toxin producing cyanobacteria was associated with a decline in Daphnia pulicaria biomass caused by a reduction in the number of individuals with a body length of >1 mm. Zooplankton biomass did not decline at moderate cyanobacteria biomass, but when cyanobacteria reached high biomass large cladocerans were reduced.
4. Our results demonstrate that zooplankton communities can be negatively affected by cyanobacterial blooms and therefore the potential to use herbivory to reduce algal blooms in such eutrophic lakes appears limited.     

Quantity and quality of seston in an impounded and a free-flowing river in Virginia,U.S.A.

Seston in the impounded North Anna River (NAR) was analyzed and compared to that in the free-flowing South Anna River (SAR) in Virginia, U.S.A. A wet filtration technique was used to separate seston into five size classes. The overall quantity of organic seston was much lower in the NAR than in the SAR. The seston in the NAR was composed of more living organisms, in particular zooplankters in the medium large (234–864 µm) and small (105–234 µm) size classes and diatoms and other algae in the fine (43–105 µm) and very fine (25–43 µm) size classes. The percentage of zooplankton declined sharply before reaching the downstream study site (32 km). The seston in the NAR tended to consist of slightly larger particles, but 80–85% of the seston in both rivers was in the ultrafine (0.45–25 µm) size class, which was almost entirely composed of detritus. One indication of seston food quality, the organic/inorganic ratio, was considerably higher for the medium large and small size classes in the NAR immediately below the dam and for the fine and very fine size classes farther downstream. Another indication of food quality, the usable caloric content, showed that the seston in the NAR contained considerably less total assimilable energy as far as 32 km downstream from the dam, but that the medium large and small size classes were rich in assimilable energy immediately below the dam because of zooplankton released from the reservoir. Thus, the overall quantity and quality of seston was lower in the NAR, but the quality of the seston, for at least a short distance below the dam, was higher in a well-defined size range that can be effectively utilized by certain filter feeders.     

Studies on the small invertebrate plankton of the Sargasso Sea

During the German Eel Expedition in Spring 1979, the horizontal and vertical distribution of the invertebrate plankton was studied in the epipelagic zone of the western central Sargasso Sea, based on 55 µm and 100 µm mesh net samples. In the isothermal waters north of the thermal front, plankton biomass was on average 2–3 times higher than in the warmer stratified waters south of the front. With regard to the fraction of small invertebrates (nauplii and microcopepods) the differences in numerical abundance between the two areas were similar to those reported in the literature for other size ranges of organisms. No divergency was obvious in the plankton composition in terms of major taxonomic groups and size classes. In both parts of the area, organisms smaller than 400 µm, which form a fraction not quantitatively sampled by the conventional 200 µm or 300 µm mesh nets, accounted for 71–92 % of the total number of organisms in the 55 µm net samples and for more than 50 % in the 100 µm net samples. Average concentrations of the potential food supply for early larval fish stages in the upper 100 m appear to be comparable with values reported in the literature for areas well known for larval fish development, such as the California Current.     

Chemical limnology of two artificial lakes used for both stormwater management and recreation

The aims of this study were to document the mainly chemical behaviour of two linked artificial lakes used for both stormwater management and recreation in the new town of Craigavon. Further, the understanding of their behaviour should help in their management and the design of other similar lakes.The lake mean total phosphorus (73 µg P l^–1), nitrate (0.50 mg N l^–1) and chlorophyll a (25 µg l^–1) concentrations, Secchi depth (1.2 m) and the estimated total phosphorus loading (1.98 g m^–2 a^–1) all classify the main lake as eutrophic. An important source of the phosphorus load on the lakes is the urban area of Craigavon (52% of the total load). The interrelationships between total phosphorus, chlorophyll a and Secchi depth in the main lake are similar to those in natural ones. In addition, the lake follows the total phosphorus load — trophic state relationships (lake total phosphorus and chlorophyll a concentrations and Secchi depth) found to apply elsewhere. These two points indicate that the artificial lakes in Craigavon behave similarly to natural ones.     

Influence of freshwater macrophytes on the littoral ecosystem structure and function of a young Colorado reservoir

The presence of rooted macrophytes, mostly the milfoil Myriophyllum sibiricum, was manipulated in enclosures in the littoral zone of a Colorado reservoir. The presence of macrophytes significantly increased the abundance of major invertebrate taxa by 70–1725% and increased the emission of methane 127%. The increase in abundance of most invertebrates was probably due to the habitat and surfaces provided by milfoil as stable isotope analyses indicated that milfoil was an insignificant carbon source for all of the invertebrate taxa, except for the milfoil midge Cricotopus myriophylli. Cricotopus is known to specialize on milfoil (other members of the genus specialize on Hydrilla or are generalists), had an isotopic signature that indicated a diet of milfoil, and was about 15 times more abundant when milfoil was present than when it was absent. Milfoil had no detectable effect on the total particulate phosphorus (TPP), soluble reactive phosphorus (SRP), total dissolved phosphorus (TDP), dissolved organic phosphorus (DOP), and Chl a of water within the enclosures. However, enclosures containing milfoil had higher concentrations of SRP in the pore water of surface sediments than enclosures that had milfoil removed. SRP in pore water dropped below 2 μg/L at >2 cm sediment depth and DOP increased progressively from nearly zero at the surface to about 150 μg/L at 15 cm depth, regardless of vegetation. Thus, milfoil had significant effects on many, but not all, measures of littoral ecosystem structure and function that were monitored.