Macrophyte development and resuspension regulate the photosynthesis and production of benthic microalgae. Carsten Lassen1, Niels Peter Revsbech & Ole ...
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Hydrobiologia 350: 1–11, 1997. c 1997 Kluwer Academic Publishers. Printed in Belgium.
Macrophyte development and resuspension regulate the photosynthesis and production of benthic microalgae Carsten Lassen1 , Niels Peter Revsbech & Ole Pedersen2 Department of Microbial Ecology, Institute of Biological Sciences, University of Aarhus, Ny Munkegade, Building 540, 8000 Aarhus C, Denmark 1 Present address: COWIconsult, Flegborg 6, 7100 Vejle, Denmark 2 Present address: Zoological Institute, The Freshwater Biological Laboratory, University of Copenhagen, Helsingørsgade 51, DK-3400 Hillerød, Denmark Received 26 November 1996; in revised form 11 March 1997; accepted 3 April 1997
Key words: resuspension, benthic microalgae, submerged macrophytes, light competition
Abstract The effect of macrophyte growth on microbenthic photosynthetic activity was studied in two large enclosures situated in a shallow, eutrophic lake. Macrophytes were allowed to develop stands of 100% coverage in one enclosure whereas they were harvested at emergence in the other. Although less than 10% of the incident light reached the benthic microphytes below the macrophytes at mid-summer, when the macrophytes reached their maximum coverage, the seasonal productivity (April–October) of the microbenthic community was still 355 g C m 2 corresponding to 65% of the productivity in the enclosure without macrophytes. Although the light attenuation by the macrophytes had a strong negative effect on microbenthic photosynthesis, the negative effect was partly balanced by increased water transparency caused by increased grazing on the phytoplankton, and the shelter provided by the plants also resulted in less resuspension. Analysis with microsensors for oxygen and scalar irradiance showed that the capacity for photosynthesis was evenly distributed throughout the uppermost 3 mm of the sediment and in the approximately 3-mm flocculent layer covering the sediment. Microbenthic photosynthesis seemed primarily limited by light. The microsensor analysis also demonstrated how conventional oxygen exchange experiments underestimate the true photosynthetic rates and indicated that more realistic rates might be obtained by measuring oxygen exchange if the exchange is facilitated by vigorous stirring. Introduction Autotrophic microbenthic communities are, on an areal basis, as productive as phytoplankton communities (Sand-Jensen & Krause-Jensen, 1997). In comparison, however, the physical and chemical environments of microbenthic and planktonic algae differ considerably (Sand-Jensen, 1989). For example, the microbenthic algae face steep spatial and temporal gradients of light, CO2 and O2 (Sand-Jensen, 1989). The impact of these gradients on photosynthesis has been studied in detail in flux chamber experiments (Revsbech et al., 1981) and by microsensors (Revsbech & Jørgensen, 1983; Revsbech et al., 1983). Most of these previous studies were conducted under controlled laboratory conditions
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with well-defined surface communities of microbenthic algae (Jørgensen et al., 1983; Revsbech et al., 1981; Revsbech et al., 1983). The general conclusion from these studies was that light and inorganic nutrient availability (including inorganic carbon) controlled microbenthic photosynthesis and thereby the primary production available for invertebrate grazers (Admiraal, 1984). Light is a key limiting factor for microbenthic photosynthesis, but several processes are responsible for absorbing light in shallow waters. Light absorption by humic substances is often important in oligotrophic systems whereas plants pigments, mainly chlorophylls and carotenoids, are the major absorbing agents in more eutrophic environments (Kirk, 1994). Hence,
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2 phytoplankton, emergent and submerged macrophytes, as well as terrestrial riparian vegetation controls the light transmission to the communities of microbenthic algae (Sand-Jensen, 1989). Light absorption by terrestrial vegetation is most important in small streams, and microbenthic primary production has been shown to correlate well with leafing and defoliation in temperate deciduous forest streams (Friberg, 1997). Emergent and submerged macrophytes, however, should be much more important as light absorbing competitors in open lowland streams and shallow lakes (Sand-Jensen, 1989). The macrophyte biomass show large temporal and spatial fluctuations and decreased microbenthic autotrophic production has been shown to coincide with macrophyte spring growth in streams (Sand-Jensen, 1989; Sand-Jensen et al., 1989). In some lakes, however, stochastic phenomenons in early spring determine whether the lake during the growth season will be dominated by phytoplankton or benthic macrophytes, but the regulating mechanisms for the outcome of the competition among phytoplankton and macrophytes are not fully understood. In coastal waters it has been shown that dense mats of benthic microalgae may regulate the flux of inorganic nutrients between the sediment and the water column (Carlton & Wetzel, 1988; Sundb¨ack & Gran´eli, 1988) and thereby limit the phytoplankton growth. Physical disturbance by highly turbulent water often causes loss of autotrophic biomass. This may be exemplified by hurricanes demolishing seagrass beds (Gallegos et al., 1992), wash out of stream macrophytes at high autumn water discharge (Sand-Jensen, 1989; Sand-Jensen et al., 1989), or grazing by cladocerans where turbulent water increases the collision rate between prey and predator (Kiørboe, 1993). In stagnant water, a loss of potential for primary production in the planktonic environment occurs when phytoplankton cells sink out of the water column and add to the microbenthic algal community where they may form a thick flocculent autotrophic layer. This layer of settled planktonic algae is easily resuspended in shallow lakes by naturally occurring wind events (Hosper & Meier, 1993). Perhaps the most negative effect of a resuspension event in terms of the production potential of the total algal community is the risk of subsequent extensive algal burial into deeper non-photic sediment strata or in deeper non-photic parts of the lake. In the present study we set out to improve our understanding of the factors controlling microbenthic photosynthesis and primary production in natural systems. We did not use experimental designs with well defined
sediment surfaces inhabited by microalgae exposed to a clear water phase. Instead we have studied a more typical freshwater system where the sediment surface is badly defined and consists of a thick flocculent and easily resuspenable layer rich in both true benthic microalgae and settled phytoplankton. We have measured the photosynthetic activity throughout a growth season in the system and described how environmental factors affect microbenthic photosynthesis and primary production. The experiments were carried out in situ on large scale enclosures where the macrophyte biomass could be controlled and manipulated, and in the laboratory where resuspension events were simulated.
Materials and methods General description of the locality Lake Stigsholm is a 21 ha eutrophic shallow lake in central Jutland, Denmark. For several years the lake has been used for management experiments dealing with the interactions between zooplankton, fish, waterfowl, and macrophytes. The experimental area comprised six ring-shaped enclosures with a 5 m wide inner ring surrounded by a 10 m wide outer ring. We sampled sediment cores from April to October 1994 in two of the inner rings. In one enclosure both the inner and outer ring were kept free of macrophytes by harvesting, whereas macrophytes in the other enclosure were allowed to grow within the inner ring. In late May, submerged macrophytes, predominantly Potamogeton pectinatus L., started growth, and in late July macrophyte growth almost completely covered the inner ring. In August, the macrophytes started to decompose and they had almost disappeared by the end of the month. The water depth in the two enclosures ranged from 48 to 53 cm. The outer ring was isolated from the lake water by a plastic membrane for two weeks in May and a period from late July to October as a part of other experiments in the enclosures. In spring, the sediment surface was covered by a flocculent layer of both planktonic and benthic microalgae and detritus. Wind-induced turbulence could suspend the particles and the flocculent layer was displaced and accumulated in sheltered areas to several centimetres of thickness. The particles were dominated by the green alga Oedogonium (filamentous, benthic) and Pediastrum (planktonic), the planktonic cyanobacteria Planktothrix and Microcystis, and various benthic diatoms. The sandy sediment below
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3 the flocculent layer was inhabited by the same algal forms. During summer, the distribution of the flocculent layer at the sediment surface became more patchy. The flocculent layer never formed a mat-like structure, but in June and July the layer was stabilised to some extent by high numbers of chironomid larvae forming tubes within the layer. Transmission of light to the sediment surface was estimated from 20 replicates measured in each enclosure with an irradiance meter (OL-2000Q, Light and Optics). Due to the geometry of the sensor the light was actually measured two cm above the sediment surface.
Production
Total flux measurements
Pigment analysis
Sediment cores were sampled in 4.2 cm I.D. Plexiglass tubes, brought to the laboratory, and incubated at in situ temperature under different irradiances from fluorescent tubes (OSRAM Fluora 75). For each irradiance the cores were pre-incubated for 1 h before measurements. Initial slopes of P/I curves were calculated from three data points in the linear range below 120 mol photons m 2 s 1 . Stirring was provided at 40 rpm by a 2-cm long magnetic stirring bar in each tube. The cores were closed with a Plexiglass cover and initial and terminal oxygen concentrations were measured with an oxygen microelectrode. For the experiment where total fluxes were compared to microsensor studies, the sediment was sampled in a 7.3 cm I.D. Plexiglass core and stirring was provided by two stirring bars. In experiments with high and low stirring, the revolution of the stirring bars was changed between 40 and 200 rpm. For each light intensity, the cores were pre-incubated for 1 h before measurements were performed, first with low and subsequently with high stirring. When the water above sediments with a flocculent layer was agitated, the particles were suspended and could be separated from the sediment with the supernatant. One experiment was performed with 6 sediment cores with a flocculent layer, 6 cores where the flocculent layer was removed, 6 cores with the flocculent layer transferred to a Chl a free sediment, and 6 sediment cores without a flocculent layer. The experiment was repeated with the flocculent layer transferred to a solid surface and these results are represented instead of the results with the layer transferred to a Chl a free sediment. The cores were incubated at low light 6 h before measurements. Initial slopes of P/I curves were calculated from three data points below 80 mol photons m 2 s 1 .
Monthly production was estimated from data on light transmission, P/I curves and meteorological data from a nearby meteorological station based on the following simplifications: (i) Light transmission through the water/air interface and through the water column was constant and the same as measured at the sampling time, i.e. independent of light incident angle and (ii) the photosynthesis/irradiance relationship was constant within the range of irradiances reaching the sediment surface.
The sediment cores were sectioned into 0–5 mm and 5–25 mm subsamples. By draining the cores before sectioning, the flocculent layer became more compact and the 0–5 mm section represents a layer of up to 10 mm in the intact sediment. About 2 g of each subsample was frozen ( 20 C), subsequently sonicated for 2 min in 40 ml 96% ethanol, extracted for 24 h in darkness, and analysed on an spectrophotometer (Hitachi 2000U) according to the method of Lorentzen (1967). The ratio, R, of absorbance before acidification to the absorbance after acidification was determined to be 1.78 using crystalline Chl a (Fluka Chemie AG). The extraction efficiency was about 90%. Oxygen microelectrodes Oxygen was measured by Clark type microelectrodes with an internal tip diameter of 2 m (Revsbech, 1989). The sensitivity to stirring was less than 1%. For O2 and photosynthesis measurements in the laboratory, the electrodes had an outer tip diameter of about 10 m and a 90% response time of 0.3–0.4 s. The electrodes were positioned by a motor-driven micro-manipulator with computerised depth control. Photosynthesis was measured by the light/dark shift method (Revsbech & Jørgensen, 1983) and calculated from the change in oxygen concentration within 0.3–1.3 s after darkening. The electrode current and the output of a photo-diode which registered the onset of darkness were recorded at 500 Hz by a PC equipped with an A/D converter. For field measurements more sturdy oxygen electrodes with an outer tip diameter of about 30 m and a 90% response time of 1 s were used. During field measurements, the electrodes were extended by a glass rod attached to a micro-manipulator above the water, and the electrode signal was registered by a strip-chart recorder.
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4 Fibre-optic microsensors Quantum scalar irradiance of photosynthetic available radiation, PAR (400–700 nm) was measured by fibreoptic scalar irradiance microsensors built according to Lassen et al. (1992A) with the modifications suggested by Lassen & Jørgensen (1994). The 70-m wide spherical collector had an isotopic response (within 10%) from 160 to 160 . The fibre-optic microsensor was via a FSMA connector coupled to a photomultiplier tube (Oriel 77344) with a wavelength range from 160–930 nm (M. K¨uhl et al., unpublished). A set of filters were placed in a unit between the photomultiplier and the connector. A short pass filter (Oriel 58891) blocked light above 700 nm and a long pass glass-plastic filter (Schott KV 399) blocked light below 400 nm. The photomultiplier had a nearly flat spectral quantum responsivity curve over the wavelength range from 400–700 nm and it was not necessary to add colour-compensating filters. For microsensor studies of PAR and photosynthesis distribution, sediment cores were placed at in situ temperature in an aquarium and illuminated vertically on the surface with a collimated light beam from a halogen lamp. By replacing the sediment sample in the experimental design with a light trap (black well), incident irradiance (i.e. downward scalar irradiance), and scalar irradiance at the sediment surface could be measured at the same position relative to the light field. Incident irradiance at the same position was measured by a LICOR underwater irradiance meter UWQ 4900. Under collimated light perpendicular to the surface, irradiance and scalar irradiance are the same and consequently the fibre-optic instrument could be calibrated from the measured incident irradiance. The photomultiplier meter was connected to a PC and the fibre-optic microsensors were positioned with the computerised depth control. At each depth the current was integrated for 3 s and with the present signal/noise ratio PAR could be measured down to 0.1 mol photons m 2 s 1 . The dark current was read by the end of each profile.
Results The importance of the submerged macrophytes The canopy formation by submerged macrophytes had a significant impact on light transmission to the sediment surface (Figure 1B). The differences between the two enclosures before macrophytes developed in late
spring were mainly due to differences in water depth. In July, the enclosure with macrophytes present developed a macrophyte cover of 100%, and only 8% of the incident light reached the sediment surface. In contrast, 37% of the incident light reached the sediment surface in the enclosure without macrophytes. During August, the light attenuation in the water column of the macrophyte free enclosure, and in the remaining part of the lake, increased due to a bloom of the cyanobacterium Microcystis aeruginosa. During this period of algal bloom both the enclosures were separated from the lake by a plastic membrane. In the enclosure with macrophytes present the water was totally clear, probably due to zooplankton grazing. In August, the macrophytes started to decompose and four times more light reached the sediment surface as compared to the enclosure without macrophytes but with plenty of planktonic phototrophs. In September, the water in both enclosures became transparent and the decomposing macrophytes had only a minor influence on the light transmission to the sediment (Figure 1B). From April to July, the initial slope ( ) of the photosynthesis versus irradiance curves of the microbenthic community within the two enclosures exhibited no significant differences. Later in August decreased within the enclosure with macrophytes present to half the rate of that in the macrophyte free enclosure and this difference remained throughout September and October (Figure 1F). During summer the formation of the macrophyte canopy had a significant effect on the microbenthic productivity (Figure 1E). From May to July the calculated productivity in the enclosure with macrophytes decreased to less than 20% of the productivity in May while the productivity in the enclosure without macrophytes remained fairly constant. During this period there was no significant differences between the value of the two enclosures, and the difference in productivity was solely due to shading by the macrophytes (Figure 1B). In late August to October, however, the differences in productivity were due to different values between the two systems (Figure 1F). During mid and late summer the macrophyte free enclosure showed a systematically higher sediment Chla concentration regardless of sediment depth (Figure 1C, D). The ratio of upper (0.5 mm) to lower (5– 25 mm) sediment strata Chl a was about 0.5 in May– June and averaged 1 in the remaining growth season. The increase of this ratio in July was purely due to a decrease in Chl a content of the deeper sediment strata. In the macrophyte free enclosure, the drop in Chl a con-
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Figure 1. Seasonal changes in enclosures with submerged macrophytes absent ( ) or present ( ) in Lake Stigsholm during April to October. (A) Temperature ( C). (B) Light transmission to the sediment surface in% of incident light at the lake surface at noon. (C) Chlorophyll (Chl a, mg m 2 ) in the upper 5 mm stratum, typically the flocculent layer. (D) Chlorophyll in the 5–25 mm sediment stratum. (E) The production (gC m 2 d 1 ) of benthic microalgae estimated from meteorological data and data in (B) and (F). (F) Initial slopes ( , mol O2 mol 1 photons 10 3 ) of P/I curves at in situ temperature. C, D and F are shown as the mean SD of ten replicates.
centration of the flocculent layer in July (Figure 1C) was due to wind-induced resuspension which displaced most of the flocculent microalgae except in areas with sheltering macrophytes. In situ oxygen profiles In situ oxygen profiles in sediments with a 3 mm flocculent microalgal layer showed pronounced diurnal fluctuations (Figure 2). At dawn, O2 penetrated less
than 1 mm into the sediment whereas at noon, O2 was detectable down to 5 mm depth and reached a maximum below the sediment surface of more than 450% of atmospheric saturation. At noon, the oxygen concentration in the entire water column reached 150% saturation. An increase in water oxygen concentration from atmospheric equilibrium and up to 150% air saturation had no significant effect on microbenthic photosynthesis (oxygen flux) in laboratory experiments, so this moderate elevation in oxygen was apparently not
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Figure 2. In situ oxygen profiles in the sediment of an enclosure with submerged macrophytes present or absent. Measurements are from early in the morning till noon, and during this period the water temperature increased from 17 to 22 C.
sufficient to cause a significant elevation of photorespiration (data not shown). Oxygen did not penetrate into the sediment beneath the flocculent microalgal layer during night-time (Figure 2). Oxygen was detectable down to 2 mm at noon beneath a 50 cm wide canopy path of submerged macrophytes (Figure 3). Above the sediment, the O2 concentration fluctuated and the data represent means of 1 min recordings. Secondary oxygen maxima deeper in the sediment, for example caused by root excretion of O2 , were not observed, and the small irregularity at 0.5–1 mm depth in the 14.45 profile was probably caused by infaunal activity. The 150% air saturation with respect to O2 within the canopy was identical to the saturation within the surrounding water derived from a combination of planktonic, microbenthic, and macrophytic photosynthesis. The effect of stirring The flocculent layer at the sediment surface was easily suspended by a stirring of the water, and during darkness oxygen penetrated to 2 mm depth into the regular sediment. When the stirring was lowered, the suspended flocs resedimented, and steep O2 gradients within the flocculent layer were reestablished within 15 minutes (Figure 4). The oxygen penetration into the 3-
Figure 3. In situ oxygen profiles in the sediment under a 50 cm wide macrophyte canopy in the afternoon at 14.45 (E ) and 15.10 (B ). The oxygen concentration of 150% of atm PO 2 within the canopy was the same as the concentration in the water column around the canopy. About 5% of the irradiance incident on the lake surface reached the sediment surface beneath the canopy.
mm thick layer of resedimented flocs was about 2 mm during darkness, so the regular sediment was completely anoxic. The sediment surface newly exposed to light by high stirring had a limited photosynthetic activity, so the depth of oxygen penetration was not increased to the same extent by light as was the case during low stirring. When the stirring rate above a sediment with a flocculent layer was increased from 40 to 200 rpm, oxygen consumption in the dark increased by a factor 5.4, but also the oxygen production rate during illumination increased (Figure 5). The initial slope of the P/I curve increased about 3 times while gross exchange rates at 280 mol photons m 2 s 1 (obtained by adding dark respiration rates to net oxygen export at 280 mol photons m 2 s 1 ) increased by a factor 2. Similar factors were obtained in sediment where the flocculent layer was removed, in flocculent particles on a solid surface, and in sediments sampled without a flocculent layer. The depth-integrated photosynthesis rates of the flocculent layer on solid surface were similar to the rates in undisturbed sediments although the Chl a content of the intact sediment was 7 times the content of the flocculent layer. The reason for this is, of course, that light was extinguished within the flocculent layer so that the microalgae in the regular sediment were inac-
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Figure 4. Oxygen profiles within and above the sediment covered by a flocculent layer of microalgae. The profiles at low stirring were measured 15 min after the change in stirring rate. Incident light was 180 mol photons m 2 s 1 .
Figure 6. Exchange rates in the sediment covered by a 5 mm flocculent layer of microalgae with a porosity of 0.95. Gross photosynthesis ( ) is shown as the mean SD of ten replicate photosynthesis profiles at each light intensity. The diffusive flux ( B ) between the sediment and the overlying water is shown as the mean SD calculated from 10 replicate oxygen profiles in the diffusive boundary layer above the sediment. The net photosynthesis rates of the entire sediment (R) were calculated from oxygen exchange rates measured at each light intensity instantly after the microelectrode measurements. Gross exchange rates (N) were calculated by adding the dark oxygen uptake rate to the net photosynthesis rates.
sediment without flocculent layer approached that of intact cores after only 4–6 h (data not shown). Gross and net photosynthesis
Figure 5. Oxygen exchange between the sediment with a 4 mm flocculent layer of microalgae and the surrounding water at high and low stirring rate. Mean SD of six replicates.
tive. When the flocculent layer was removed from the sediment, oxygen production in the sediment started immediately due to the presence of light. The microalgae apparently recovered rapidly from their life under non-photic conditions, as the integrated activity of the
The apparent increase in gross photosynthetic rates with higher stirring may be a result of an underestimation of the gross photosynthesis rates at low stirring because a significant part of the produced oxygen is consumed by an increased oxic respiration within the sediment. Gross photosynthesis rates measured with microelectrodes were compared to net photosynthesis measured as total oxygen exchange at low stirring (flow rates of about 1 cm s 1 ) (Figure 6). At 290 mol photons m 2 s 1 the gross rates were 2.1 times higher than gross exchange rates but the initial slope of the P/I curve (below 180 mol photons m 2 s 1 ) increased only by a factor of 1.3. The respiration, calculated as the difference between gross rates and net exchange, increased 3.6 times. In the dark, the diffusive flux comprised only 39% of the total flux whereas it comprised 73–83% of the total flux in light. The flocculent layer
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Figure 7. Depth distribution of oxygen, gross photosynthesis, and scalar irradiance (PAR) in a sediment without a flocculent layer (A) and in a sediment with a 4 mm flocculent layer of microalgae (B). Incident irradiance was 175 mol photons m 2 s 1 . Photosynthesis and PAR are shown as the mean of ten replicates, and O2 data represent the mean of 3 replicates.
was inhabited by a high number of chironomid larvae creating irrigated burrows at the time of the experiment. Light and photosynthesis distribution Light and photosynthesis distribution were not systematically different in bare sediments and in sediments with a flocculent layer of microalgae present (Figure 7A, B). The apparent deeper O2 penetration in flocculent sediments was not significant as replicate measurements showed some instances of O2 penetration down to 6.5 mm in bare sediments (data not shown). The optical characteristics for the upper 3 mm stratum were also comparable in the two types of sediments with attenuation coefficient (E0 ) of 2.0 mm 1 in the flocculent sediment and 1.7 mm 1 in the bare one. The photosynthesis versus irradiance rates (P vs I) for the various depths analysed in Figure 7B were plotted in Figure 8 in addition to the P vs I values for two additional cores. The data were fitted (r2 = 0.88) to
Pgross = Pmax
I Pmax + I
and showed a high -value of 0.051 nmol O2 cm 3 s 1 (mol photons m 2 s 1 ) 1 in the low light region (
