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The effect of submerged macrophytes on interactions among epilimnetic phosphorus, phytoplankton, and heterotrophic bacterioplankton has been acknowledged, but remains poorly understood. Here, we test the hypotheses that the mean summer phytoplankton biomass (chlorophyll a): phosphorus ratios decrease with increased macrophyte cover in a series of nine lakes. Further, we test that both planktonic respiration and bacterioplankton production increase with respect to phytoplankton biomass along the same gradient of increasing macrophyte cover. Increased macrophyte cover was associated with a lower fraction of particulate phosphorus in epilimnia, with total particulate phosphorus declining from over 80% of total phosphorus in a macrophyte free lake to less than 50% in a macrophyte rich lake. Phytoplankton biomass (chlorophyll a) too was lower in macrophyte dominated lakes, despite relatively high levels of total dissolved phosphorus. Planktonic respiration and bacterioplankton production were higher in macrophyte rich lakes than would be expected from phytoplankton biomass alone, pointing to a subsidy of bacterioplankton metabolism by macrophyte beds at the whole lake scale. The results suggest that the classical view of pelagic interactions, which proposes phosphorus determines phytoplankton abundance, which in turn determines bacterial abundance through the production of organic carbon, becomes less relevant as macrophyte cover increases.

As human activities continue to alter the global nitrogen cycle, the ability to predict the impact of increased nitrogen loading to freshwater systems is becoming more and more important. Nitrogen retention is of particular interest because it is through its combined processes (denitrification, nitrogen sedimentation and uptake by aquatic plants) that local and downstream nitrogen concentrations are reduced. Here, we compare the magnitude of nitrogen retention and its components in wetlands, lakes and rivers. We show that wetlands retain the highest proportion of total nitrogen loading, followed by lakes and then rivers. The differences in the proportion of N retained among systems is explained almost entirely by differences in water discharge. Denitrification is the primary mechanism of nitrogen retention, followed by nitrogen sedimentation and uptake by aquatic plants.

Bacterioplankton and total planktonic community respiration were measured in 14 Canadian lakes during the summer. Total planktonic community respiration (O2 consumption and CO2 production) was best predicted by total phosphorus (TP), with respiratory rates increasing at higher TP levels. The bacterioplankton respiration alone was less well linked to nutrient concentrations. Yet, the rates of both planktonic and bacterial CO2 production changed similarly with changes in the C:N ratio, with the bacterial fraction, on average, contributing about 42% of the total planktonic respiratory rates. Bacterial carbon respired was uncoupled from oxygen consumption, with resultant average planktonic respiratory quotient (RQ) close to 2. Bacterioplankton respiratory rates are affected by a negative relationship between specific aerobic respiration and bacterial biomass. Higher bacterial specific oxygen consumption rates (µg O2 cell-1 d-1) were observed with increasing C:N and C:P ratios, suggesting an increase in maintenance cost at the low cell densities observed in oligotrophic waters. Phosphorus appeared as the limiting nutrient in these lakes and determined total planktonic respiratory rates, however, epilimnetic bacterial respiration was mainly related to the DOC: nutrient ratios.

The utility of biomass density (biomass per unit volume) as a quantitative descriptor of the growth form of submerged macrophytes is evaluated and confirmed. Biomass density (BD) is a species specific characteristic which does not appear to be influenced by plant development. Secondly, BD reflects the growth form of the dominant species in the stand, because stands of species with a similar growth form also have similar BD's. Lastly, the BD of submerged macrophyte stands is closely related to the flowering pattern of the dominant species, and has important implications for light capture and light competition with neighbouring plants. Thus, BD should be a useful tool in the quantitative analysis of submerged macrophyte community structure.

Two of three Kenyan lakes studied between November 1979 and October 1980 have very short 33PO4 turnover times, indicating a high phosphorus (P) demand throughout the year. The P turnover time in Lakes Oloidien and Sonachi is as rapid as in the most P deficient temperate zone lakes. The third lake, Lake Naivasha, has a lower overall P demand and a wide seasonal range, with lowest demand between November 1979 and February 1980 when a P deficiency was unlikely. On an annual basis the Lake Naivasha status is, however, not statistically different from that recorded during the summer in Lake Memphremagog, a generally P-limited temperate zone lake. Lake Naivasha and Lake Oloidien fit well to the line of best fit for the Dillon-Rigler relationship relating total phosphorus (TP) and chlorophyll a derived in temperate zone lakes. Thus, temperate zone models predicting aspects of lake behaviour on the basis of TP may also be applicable to these two tropical lakes. Saline lake Sonachi had not only a short P turnover time but also responded dramatically to the fertilization of enclosures with P. However, it does not fit the TP-chla or the total nitrogen-chla plots from the temperate zone. This suggests that, in this saline lake at least, much of the TP is unavailable to the algae, with some of it in a particulate form that is readily extracted with boiling water. The epilimnetic N:P ratios also characterize lakes Oloidien and Sonachi lakes as highly P deficient and lake Naivasha as more moderately P limited. A single set of measurements in Winam Gulf (Lake Victoria) also showed a rapid P turnover time and thus P limitation, but as in lake Sonachi much of the TP was in a non-algal particulate form. Occasional measurements in three other hypertrophic and saline lakes suggest them to be primarily light limited on the basis of their very high photosynthetic cover. These findings support the hypothesis of a primary P limitation for those lakes not light limited, and contradicts literature suggestions that nitrogen is the primary limiting element in tropical lakes.

Seasonal changes in the epiphyte biomass, measured both as chlorophyll a and as cell volume, and species composition were compared on Potamogeton richardsonii and on a similar plastic plant ‘grown’ together in a shallow bay of Lake Memphremagog (46°06'N, 72°16′W). Both substrates exhibited two periods of high biomass during the June to September growing season; one in June, when the community was dominated by loosely attached species with a strong planktonic component (up to 37%), and one in September, when the epiphytes were characterized by species tightly attached to the leaves. Although this seasonal trend was similar, the loosely and tightly attached communities were best developed on the natural and artificial plants, respectively. The diversity of the epiphytes was significantly higher on the natural than on the artificial leaves from July on. Both the diversity differences and differences in community structure appear to be the result of the summer accumulation of CaCO3 observed only on the upper leaf surfaces of the natural plants. Consequently, P. richardsonii appears to affect epiphyte development largely by its precipitation of CaCO3, with no evidence for either direct inhibition or stimulation of the epiphytes by the natural plants. The reduced epiphyte biomass on growing tips was no different from that on artificial plants of the same age and exposure and is attributable to an insufficient time for colonization rather than to inhibition by the macrophyte.