Seasonal Dynamics of Benthic and Planktonic ... - Wiley Online Library

Internat. Rev. Hydrobiol.

90

2005

1

1–20

DOI: 10.1002/iroh.200410737

PETRA WERNER and JAN KÖHLER* Institute of Freshwater Ecology and Inland Fisheries, Müggelseedamm 301, 12587 Berlin, Germany; e-mail: [email protected]; [email protected].

Seasonal Dynamics of Benthic and Planktonic Algae in a Nutrient-Rich Lowland River (Spree, Germany) key words: river ecology, periphyton, chlorophyll a, phytoplankton, diatom assemblages

Abstract We studied chlorophyll a (chl. a), biovolume and species composition of benthic algae and phytoplankton in the eutrophic lower River Spree in 1996. The chl. a concentration was estimated as 3.5 (2.7–4.5) µg/cm2 for epipsammon, 9.4 (7.4–11.9) µg/cm2 for epipelon and 6.7 (5.7–7.8) µg/cm2 for the epilithon (median and 95% C. L.). The mean total biomass of benthic algae was significantly higher (6.0 µg chl. a/cm2) than the areal chl. a content of the pelagic zone (1.6 µg chl. a/cm2). Although certain phytoplankton taxa were abundant in the periphyton, benthic taxa generally dominated the assemblages. Seasonal dynamics of benthic algae were probably controlled by light and nitrate supply (sand), discharge fluctuations (sand, mud) and invertebrate grazing (stones). This paper shows the importance of benthic algae even in phytoplankton-rich lowland rivers with sandy or muddy sediments.

1. Introduction Periphyton is important for the water quality of a river. For example, growth and decomposition of benthic algae causes fluctuations in oxygen content. Also, benthic algae may decrease nutrient levels in a river system, because they use nutrients from both the pelagic and benthic environments (BORCHARDT, 1996; TUCHMAN, 1996). For example, their ability to use nutrients from the sediment may deplete this nutrient pool before it reaches the water column (STEVENSON, 1996). BIGGS et al. (1998) and BIGGS (1996) suggest that the most important factors limiting periphyton abundance in rivers are: 1. A high frequency of flooding events (e.g. BIGGS, 1995; VELASCO et al., 2003; MÜLLNER and SCHAGERL, 2003). Periphyton biomass is reduced due to decreased substrate stability with increasing flooding frequency (e.g. PETERSON, 1996). Also, during flooding, algae may be torn from their substrate due to the current or damaged by particles contained in the current. 2. Deficiencies in nutrient and light availability (e.g. DENICOLA and MCINTIRE, 1990; STEVENSON et al., 1991; WELLNITZ et al., 1996). 3. Grazing pressure from invertebrates. However, grazing is usually considered to be less important than other environmental parameters such as 1. and 2. (e.g. KJELDSEN, 1996; STEINMAN, 1996). The abundance of periphyton is site- and season-specific, because the above factors differ through space and time. Also, substrate type and availability are important factors influencing benthic algae in a river. Most studies to date have focused on epilithon (review see * Corresponding author

© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1434-2944/05/102– 0001

2

P. WERNER and J. KÖHLER

ROUND, 1991; also e.g. BIGGS, 1995; WINTER and DUTHIE, 1998; CHÉTELAT et al., 1999) or on artificial substrate (e.g. DENICOLA and MCINTIRE, 1990; STEVENSON et al., 1991; WELLNITZ et al., 1996). Epipsammon and epipelon were, by comparison, studied in few rivers (RIER and KING, 1996; SAND-JENSEN et al., 1988; KJELDSEN et al., 1996). In lowland rivers, most studies on algae focus on phytoplankton (review see REYNOLDS and DESCY, 1996; also e.g. KÖHLER, 1994; YANG et al., 1997; EVERBECQ et al., 2001), as phytoplankton is considered an important primary producer according to the River Continuum Concept in the lower stretches of a river (VANNOTE et al., 1980). The role of periphytic algae in lowland rivers is still largely unknown, despite some investigations of benthic algae in this river stretch (KJELDSEN, 1996; CHÉTALAT et al., 1999; SAND-JENSEN et al., 1988). No comparisons were made between benthic and planktonic algae. This study was undertaken to identify 1) the importance of algal periphyton in comparison to phytoplankton and 2) the factors driving changes in the periphytic algae abundance in the River Spree, a eutrophic, phytoplankton-rich lowland river. Therefore, our main objective was to quantify epipsammon, epipelon, epilithon and phytoplankton biomass using a) chlorophyll a measurements throughout the year and b) the Utermöhl technique and the identification of dominant algae (diatoms) under higher magnification for one spring and one summer sample.

2. Methods 2.1. Site Description The River Spree is about 380 km long and drains an area of ca. 10,000 km2. It flows through several reservoirs and shallow lakes. Thus, the River Spree is rich in phytoplankton and nutrients (Table 1, for more details see KÖHLER, 1994). The algal periphyton of sand, mud, and stones were sampled from the lower River Spree, close to Freienbrink, Germany (river-km 322, Fig. 1). The phytoplankton was sampled at Neu Zittau (ca. 10 km downstream from Freienbrink), while the epilithon was sampled ca. 500 m downstream from the epipsammon and epipelon. At the sampling sites, the River

Table 1.

Pelagic descriptive statistics of physical, chemical and biological parameters of the River Spree close to Neu Zittau, 1996.

Parameter

Unit

mean

minimum

maximum

Slope Discharge Mean water velocity Temperature pH Conductivity Total phosphorus Soluble reactive phosphorus (SRP) Dissolved nitrogen Nitrate Dissolved silicon Oxygen Oxygen saturation Seston content (dry weight) Phytoplankton chlorophyll a Phytoplankton biovolume

% m3/s m/s °C

0.01 13.78 0.47 12.4 7.6 680 113 46 0.99 0.93 4.0 7.8 72 6.7 12.4 2.1

3.83 0.40 0.4 7.3 620 72 12 0.33 0.30 1.4 4.0 50 2.0 2.2 0.3

32.30 0.61 21.6 8.0 729 168 87 2.27 1.62 6.0 11.9 89 14.8 48.4 7.6

µS/cm µg/l µg/l mg/l mg/l mg/l mg/l % mg/l µg/l mg/l

© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

3

Benthic and Planktonic Algae in a Lowland River

Table 2. Location of the sand, mud and stone sites in the River Spree and water velocity above the substrates on sampling dates (Freienbrink, Germany, 1996). Water velocity was measured in 50 cm water depth above sand and mud and directly above the stone substrate.

Distance to shore (m) Water depth (m) Water velocity (m/s)

sand

mud

stones

1.1–4.0 0.75–1.61 0.17–0.55

0.1–1.05 0.2–1.01 1.0–0.12

0.15–1.5 500) cells were counted and measured with an inverted microscope under 400 X magnification. Cells damaged more than 10% were not counted. If a chloroplast was present, the cell was counted as alive when sampled. The biovolume (mm3/cm2) was calculated using simple geometric figures for the differentiated taxa (Appendix 2A, e.g. EDLER, 1979). The third dimension often had to be estimated (Appendix 2B).

2.4.3. Diatoms The organic material from sandless sediments and all other samples (1–5 ml) was digested using hydrogen peroxide (H2O2; end concentration 25% v/v). The samples were washed four times with water (centrifugation: 1500 rpm for 10 min) and mounted in Naphrax® for identification under 1250 X magnification. At least 200 valves were counted, identified and measured. All cells were counted unless they were damaged by more than 10%, in girdle band view, or covered with particles. The biovolumes were calculated as described for the Utermöhl samples (see above). Taxonomy is based on KRAMMER and LANGE-BERTALOT (1986–1991).

3. Results 3.1. Chlorophyll a (chl. a) The sand and mud sediments and epilithon samples contained chl. a levels of up to 70 µg/cm2. The chl. a content was estimated as 3.5 (2.7–4.5) µg/cm2 for epipsammon, 9.4 (7.4–11.9) µg/cm2 for epipelon and 6.7 (5.7–7.8) µg/cm2 for the epilithon (median values and 95% confidence limits, Fig. 3). The mean chl. a concentration of each substrate was highly significantly different from that of the other substrates (Tukey-HSD test, p < 0.001). The chl. a content was influenced by both sampling date and substrate type (two-way ANOVA, p < 0.001). All three substrates had their chl. a maximum in April. The chl. a contents of the



7

25 a) sand 20 15 10 5 0 1. Apr. 80

31. Mai.

30. Jul.

28. Sep.

27. Nov.

28. Sep.

27. Nov.

28. Sep.

27. Nov.

chlorophyll a [µg/cm2]

b) mud 60

40

20

0 1. Apr. 25

31. Mai.

30. Jul. c) stones

20 15 10 5 0 1. Apr.

31. Mai.

30. Jul.

Figure 3. Chlorophyll a (solid line = mean) of each sampling date of epipsammon (a), epipelon (b) and epilithon (c) of the lowland River Spree 1996. Dashed line = chlorophyll a content of the water column above the substrates, with minimum and maximum boundaries for the sand. The minimum is not displayed for the mud and stone substrate, because they were in shallower water, i.e. the minimum was close to zero.

sand sediment and epilithon increased again at the end of the year. In contrast, the chl. a content of the mud remained relatively constant at a lower level for the rest of the year. The chl. a values of epipsammon, epipelon and phytoplankton as well as ammonium and discharge fluctuations were log transformed. The remaining variables used for the following statistics all showed a normal distribution. The chl. a content of the sand increased significantly with higher water velocity (r = 0.66, p < 0.05), increasing dissolved nitrogen and nitrate concentrations (NO3) as well as nitrate load (discharge * NO3; r = 0.58, 0.69 and 0.72, © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

8


Table 3. Pearson correlation matrix for seasonal chlorophyll a distribution (n = 17) of epipsammon, epipelon, epilithon and phytoplankton in relation to environmental variables. Chlorophyll a (chl. a) values for epipsammon, epipelon and phytoplankton as well as ammonium and discharge fluctuations were log transformed. The remaining variables were normally distributed without transformation. Discharge fluctuations = the difference between the minimum and maximum of discharge levels from within the two weeks before the sampling date; * α < 0.05; ** α ≤ 0.001. Epipsammon r Chl. a epipelon Chl. a epilithon Chl. a phytoplankton Phytoplanktonic biovolume Seston content Soluble reactive phosphorus Dissolved phosphate Dissolved nitrogen Ammonium Nitrate (NO3) Discharge * NO3 Discharge fluctuations Mean water velocity Dissolved silicon Oxygen

Epipelon r

Epilithon r

0.30 0.41 0.48 0.49* –0.56* –0.58* 0.44 –0.05 0.44 0.50* –0.07 0.21 –0.58* –0.06

–0.01 0.21 0.15 –0.59* –0.59* 0.48 0.31 0.51* 0.54* 0.06 0.28 –0.15 0.68*

0.39 0.29 –0.36 –0.12 –0.30 –0.33 –0.40 0.58* –0.23 0.69* 0.72** –0.61* 0.66* 0.29 0.40

Phytoplankton r

0.78** 0.95** –0.46 –0.45 –0.37 –0.11 –0.29 –0.26 0.54* –0.54* –0.88** –0.37

respectively; Table 3, Fig. 4c). Also, the chl. a content in the sand decreased significantly with increasing chl. a content in the pelagic zone (r = 0.57, p < 0.05; Fig. 4b) and larger discharge fluctuations (r = –0.61; Fig. 4a). Similarly, the chl. a content of epipelon and epilithon increased significantly with increasing nitrate load (r = 0.50 and 0.54, respectively). The chl. a content of the mud also increased with increasing seston content of the pelagic zone (r = 0.49), which was closely related to phytoplankton chl. a (r = 0.95; Table 3). The pheopigment concentrations averaged at 61– 65% of the chl. a for the three substrates but at only 48% of the phytoplankton chl. a. Based on the sediment description from the 6.6 km river stretch upstream from Freienbrink (FISCHER and PUSCH, 2001), we estimated a mean benthic algal biomass of 6.0 µg chl. a/cm2 for this river section in 1996. This value is significantly higher than the mean areal chl. a content of the pelagic zone (1.6 µg chl. a/cm2) for 1996 (paired, 2-sided t-test, p < 0.001; Fig. 3). 3.2. Biovolume Similar to the chl. a content, the biovolume of alive benthic algae was higher than that of phytoplankton in the overlaying water column in six of the seven samples from May 15 and June 26, 1996. The sand samples had the lowest living biovolume (0.11 to 1.77 mm3/cm2) and the mud samples the highest (up to 18.31 mm3/cm2). In general, the biovolume was higher in May than in June, except in one of the two June sand samples (Fig. 5). In contrast, the biovolume in the pelagic zone was slightly lower on May 15th than on June 26th and reached only up to 0.86 mm3/cm2 during this time period. Corrected for the relative abundance of the sediment types, the biovolume of the planktonic taxa was 2.2 times


9

Benthic and Planktonic Algae in a Lowland River 25 20

10 8

15 6 10 4 5 0 1-Apr

21-May

10-Jul

29-Aug

18-Oct

0 7-Dec

50

200 chl. a of pelagic zone water depth Secchi depth

45 40

180 160

35

140

30 25

120 100

20

80

15

60

10

40

5

20

0

0 1-Apr

sand chl. a [µ µg/cm2]and Q x NO3 [µ µ g*l/sec.]

c)

water- and Secchi-depth [cm]

µg/cm2] and chl. a of sand [µ µg/l] pelagic zone [µ

b)

2

discharge changes [m3/sec.]

12 sand chl. a discharge changes date sampled for biovolume

21-May

10-Jul

29-Aug

18-Oct

7-Dec

25 Q x NO3 pelagic zone NO3-N

20

1800 1600 1400 1200

15

1000 800

10

NO3 [µ µg/l)

sand chl. a [µ µg/cm2]

a)

600 400

5

200 0

1-Apr

21-May

10-Jul

29-Aug

18-Oct

7-Dec 0

Figure 4. Mean sand chlorophyll a (chl. a) content in the River Spree in 1996 (n = 1–3, mainly three) compared to a) discharge fluctuations (the difference between the minimum and maximum of discharge levels from within the two weeks before the sampling date); discharge ranged from 3.8 to 16.3 m3/s; b) the phytoplankton chlorophyll a levels, water and Secchi depth (lacking data = Secchi depth exceeded water depth) and c) nitrate (NO3) passing over the sand (discharge (Q)*nitrate) and nitrate levels.

higher on the sediments than in the pelagic zone on May 15th, but only 1.35 times higher on June 26th (Fig. 5). In the analysed samples, 40% (mud May), 69% and 84% (both sand June) of the cells were considered alive (chloroplast present). In the remaining four samples from May 15th and June 26th 1996, 53 – 55% of the cells were alive (Fig. 6a).


10

P. WERNER and J. KÖHLER 20

20

15

15 10 10

biovolume [mm3/cm2]

chlorophyll a [µg/cm2]

25

5

5 0

sand-a

May

sand

0

sand-b

June

May

June

mud

May June

stone

substrate type

May

June

pelagic zone

Figure 5. Chlorophyll a concentration (black bar) and biovolume (alive; grey) of benthic algae in the River Spree close to Freienbrink (Germany) on May 15 and June 26, 1996. Benthic algae are also compared to areal chlorophyll a and biovolume (alive) at Neu Zittau (calculated for the mean water depth of the river course). Pelagic zone biovolume is an average of May 7 and 21 (May) and of June 18 and July 2 (June). Darker grey bar: Phytoplankton contribution to the total biovolume (all nondiatoms, Asterionella formosa and centric diatoms excluding Aulacoseira spp. and Melosira spp.).

Diatoms (class Bacillariophyceae) contributed 94.6 ± 1.7% to total algal periphyton biovolume (alive and dead cells, ±C.L.) of all substrates on May 15th and 88.4 ± 4.5% on June 26th. Both the stone and sand-b June samples contained a high proportion of centric biovolume, whereas the remaining sand and mud samples were dominated by pennates. In June, sand and stone samples had less Melosira and more Aulacoseira biovolume than in May (Fig. 6a). These patterns are also apparent in the diatom counts (see below; Fig. 6b). Similarly to the periphyton, the diatoms contributed a larger share to the phytoplankton in May (78–93%) than in June (49 – 73%). The remaining phytoplankton biovolume was mainly composed of cryptophytes (up to 10%) and green algae (up to 40%). Among the green algae, Oocystis spp., Scenedesmus spp. and Siderocelis spp. were most important. In the algal periphyton, non-diatoms made up only 2% (sand May) to 20% (stone June) of the biovolume (alive; Fig. 6a). Non-diatoms mainly consisted of Pediastrum spp. and Scenedesmus spp. (both class Chlorophyceae). 3.3. Diatom Assemblages The diatom assemblages from the seven algal periphyton samples were comprised of 89 taxa on May 15th and June 26th, 1996. Only 17 taxa made up more than 3% biovolume in any one of the samples, of which 11 taxa occurred on all substrates (Fig. 6b). The biovolume of the benthic taxa combined was higher in May (up to 77%; sand) than in June (as low as 10%; stone) for all three substrates (Fig. 7). On the sand Gyrosigma attenuatum, Navicula reinhardtii and Nitzschia sigmoidea contributed together up to 56% (May; Fig. 6b). Of the benthic diatom biovolume, 86% (May) to 96% (sand-a, June) were potentially motile (bi- or monoraphids) in the sand, compared to 46 – 60% on the mud and


11

Benthic and Planktonic Algae in a Lowland River no n no -dia n- t o G di m yr at s o a pe osig ms liv nn m d e at a a ead es li al ve iv (0 pe e % nn de at ad es ) M d ea el d M osi r A elos a al u A lac ira d ive u o e ce lacoseir ad n s a ce tric eira ali nt s a d ve ric li ea s d ve d ea d

tychoplankter plankter

benthics

a) sand May sand-a June sand-b June mud May mud June stone May stone June

20

20 20 40 60 20 40 20

20

20 20 40

relative biovolume [%] from Utermöhl technique tychoplankter

G yr

b)

plankter

os i N g ma itz at sc hi ten u as N av ig atum m Co icu o id G cco la r ea om n ei G ph eis nha y r o p rd ot os ne ed ti he ig m ic i Co r bema a aculus c n no u Fr con thic dif min ag ei s er at um um Fr ila s p a r l M gila ia c ace e r o nt ot losi ia u nstr ula h r l A er a v na/ uen ul ty ar ac s a c i A cos hop ans us ul ei la ac ra n os a kt ei mb o n ra ig ot he gr ua A r an c t pl ul in an at o a Cy cy kte cl clu r os s t Cy eph nor a m c St lote nos ani ep ll d i St han a ra ubiu . i o di s St nvi dis osa . n sit cu eo atu s h as s an tra tz sc ea hi i

benthics

sand May sand-a June sand-b June mud May mud June stone May stone June pelagic zone May pelagic zone June 20

20

20

20 20

20

20

20 40

20

20

20

20 40 60

relative biovolume [%] from diatom counts

Figure 6. Relative biovolume of algae from sand, mud and stone substrate from the River Spree close to Freienbrink, from a) the Utermöhl technique and b) the diatom counts from May 15th and June 26th, 1996 and phytoplankton in May (average from May 7th and 21st) and June (average of June 18th and July 2nd). Each row adds to 100% relative biovolume. Only taxa that occurred at least once to 3% are displayed.

stone samples (Fig. 7). Four of the abundant benthic taxa occurred on all three substrates: Cocconeis placentula contributed up to 17% to the biovolume (sand-a in June), and Fragilaria construens up to 3% (mud in May). Interestingly, Fragilaria ulna formed up to 4% of the biovolume in May but was absent in all June samples. Lastly, Melosira varians made up a large share of the biovolume in the mud and on the stone (up to 11% in May), but only contributed up to 5% in the sand (Fig. 6b).


12 benthic diatoms [% of biovolume]


100 80 60 40 20 0

May

sand-a

sand

June

sand-b

May

June mud

May

June stone

Figure 7. Relative biovolume of benthic diatoms on sand, mud and stone substrates (from the diatom counts) on May 15th and June 26th 1996 in the River Spree. Grey: possibly motile benthic diatoms (monoand biraphids), black: immotile (araphid) benthic diatoms.

The 5 tychoplanktonic taxa made up 3 –13% of the biovolume in the seven samples. Aulacoseira ambigua and A. granulata together contributed 1– 8% to the biovolume in May, but 13–17% to each sample in June, with the exception of sand-b in June (5%) (Fig. 6b). The 12 planktonic taxa made up 20 – 60% of the biovolume in the sand (except for sand-b in June: 76%) and mud samples, but 66 –74% in the stone samples. The high contribution of phytoplankton to the biovolume was mainly due to Stephanodiscus neoastraea, which made up 11 – 27% of the sand and mud samples and 30 – 44% of the stone samples. Cyclostephanos dubius represented only 0.4 – 9% of the sand samples (exception sand-b June, 26%), but 8 – 21% of the remaining mud and stone samples (Fig. 6b). In the pelagic zone, the relative diatom biovolume followed a similar pattern to the planktonic taxa contained in the periphyton in 1996, as Stephanodiscus neoastraea and Cyclostephanos dubius dominated the phytoplankton community (up to 56% and 16%, respectively). Also similar to the periphyton assemblages, Aulacoseira spp. and Actinocylus normanii subsalsus both increased from May (4% and 5%) to June (35% and 8%; Fig. 6b) and the relative abundance of benthic taxa decreased (from 9% to 2%). In contrast, Cyclostephanos dubius decreased in the phytoplankton from May to June, while it increased on all substrates.

4. Discussion 4.1. Indication of Trophic Status According to BIGGS’s (1996) classification of nutrient status of rivers based on epilithon chl. a values, the River Spree is identified as a ‘nutrient rich’ system (Table 4). This agrees with nutrient concentrations of the River Spree, where total phosphorus values averaged at 113 µg/l in 1996 (Table 1). Also, the species assemblages found on the substrates and in the phytoplankton reflect the high nutrient and high conductivity levels (>500 µS/cm). For example, Aulacoseira ambigua, A. granulata, and Stephanodiscus neoastraea indicate eutrophic conditions (KRAMMER and LANGE-BERTALOT, 1991). Nitzschia sigmoidea and Cocconeis pediculus prefer medium to high conductivity levels (KRAMMER and LANGE-BERTALOT, 1988 and 1991).



13

Table 4. Periphyton chlorophyll a-content in rivers, streams and estuaries from several studies covering the time span of one year (values in µg/cm2). * values show the high and low quartile and median. substrate

min.

max.

Mean

comments

Mud sediment Fine grain sediment

1.95 1.8

69.6 95

9.4*

Clay, sand and gravel sediments Six different estuary sediments

~0.1

50

results of this study values from 3 years, eutroph lowland river (KJELDSEN et al., 1996) eutrophic lowland river; 0.5 cm substrate depth (SAND-JENSEN et al., 1988) 0.5 cm substrate depth; freeze drying; values from 3 years (JONGE and COLIJN, 1994) 3 cm substrate depth; freeze drying (NEUMANN, 1995) results of this study eutroph river (RIER and KING, 1996) results of this study In 2 years; eutroph (KJELDSEN et al., 1996) 1987; lowland river (KJELDSEN et al., 1996) eutrophic river (RIER and KING, 1996) nutrient poor rivers (BIGGS, 1996) medium nutrient content (BIGGS, 1996) nutrient rich rivers (BIGGS, 1996) 13 lowland rivers, summer (CHÉTELAT et al., 1999)

Sediment

2.9–24.7 9

40

24

0.32 1.8 1.67 3.2

21.3 5.1 21.2 32.8

3.5*

Stones

8

220

Stones Stones Stones Stones Stones

7.2 0.05* 0.3* 2.5* 0.9

18.1 0.3* 6.0* 26.0* 47

Sand sediment Sand sediment Stones Stones

6.7*

0.17* 2.1* 8.4*

4.2. Substrate Stability The extend of algal removal through discharge fluctuations will decrease with substrate stability (PETERSON, 1996). Therefore, the epipelon and epipsammon were likely the most influenced by discharge changes. The discharge was relatively constant in the six days before the May sampling (15 –16 m3/s). On the sand, it appears likely that there was a succession concurrent with a biomass increase before May 15th. This is supported by the relatively high chl. a content on May 15th (6.0 µg/cm2, 1996 median chl. a sand content was 3.5 µg/cm2). Also, large, and therefore easily breakable, taxa like Fragilaria ulna, Gyrosigma attenuatum, and Nitzschia sigmoidea (together contributing 57% to the May diatom biovolume) point towards high substrate stability, because these cells would otherwise be quickly destroyed by moving sand grains (DELGADO et al., 1991; STEVENSON, 1996). During summer, low chl. a content on sandy substrates coincided with the largest and most frequent water level fluctuations (Table 3, Fig. 4a). This indicates a possible limitation of biomass accumulation through substrate instability on the sand and thus resuspension or mechanical damage of the epipsammon. For example, two weeks before June 26th, the discharge fluctuated drastically between 4 –12 m3/s. Therefore, the high percentage of alive diatoms on the sand on June 26th can be explained by high substrate disturbance, because dead matter would be washed away. In addition, more than 86% of benthic diatoms were motile on the sand. High disturbance rates should favour motile species, which can migrate back to the light after they got buried (review see BURKHOLDER, 1996). For example,


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Navicula reinhardtii (15% diatom biovolume of sand-a) is motile and heavy enough to not get washed away, but is considerably smaller than the big pennates in May, i.e. less likely to break due to sand movement. 4.3. Growth and Nutrient Concentration Many studies show no clear pattern of which nutrients (nitrogen or phosphate) are limiting and at which levels, partly due to other limiting factors overriding nutrient limitation. Several studies have shown that 100 µg NO3–N/l and possibly higher levels can limit algal periphyton growth (review BORCHARDT, 1996). Also, some of these studies suggest that nutrient limitation may especially occur in early stages of succession and become less important in a well developed periphyton mat, when internal nutrient recycling becomes more important (e.g. STEVENSON et al., 1991; STELZER and LAMBERTI, 2001). In our study, sand chl. a was lowest during the summer, when concentration of inorganic nitrogen was lowest in the pelagic zone (≤760 µg NO3–N/l; Fig. 4c) and water velocity was relatively slow (Table 3). Therefore, low exchange rates between sediment surface and water might have caused nitrogen limitation of epipsammic algae. At least, we found a significant positive correlation (r = 0.72, p = 0.002) between nitrate load (nitrate concentrations times discharge) and sand chl. a levels (Fig. 4c). Other nutrients such as silicon and phosphorus were not significantly correlated to epipsammic chl. a (Table 3). Nitrate load was also positively correlated with the chl. a content of the epipelon and epilithon (Table 3). However, nutrient limitation of algal growth on stones and mud is very unlikely. The low water velocity (< 0.12 m/s and < 0.01m/s, respectively) above these substrates allows for an intense settlement of organic matter. Therefore, nutrient supply of epilithic and epipelic algae was probably less influenced by external concentrations because of high levels of nutrient recycling within the biofilm. High remineralisation rates in the benthos of the River Spree are supported by the presence of very high bacterial production, even in the sandy sediments of the River Spree, compared to other river systems (FISCHER and PUSCH, 2001; FISCHER et al., 2002). 4.4. Growth and Light Supply Epipsammon growth was probably limited by light supply. This assumption is supported by the negative relationship between the chl. a content of the sand and chl. a content in the pelagic zone (i.e. chl. a content decreased significantly in the sand while it increased in the pelagic zone, Fig. 4b). Light limitation by shading through phytoplankton is further substantiated by the fact that the Secchi depth was lower than the water depth only in June/July 1996, when the chl. a content on the sand was the lowest (Fig. 4b). Beside high phytoplankton concentrations, light intensity was further reduced by resuspended organic matter on the 0.75 –1.6 m deep sandy substrates. Resuspension was probably highest during summer due to frequent water level fluctuations. Therefore, in summer, biomass-specific photosynthesis rate of the epipsammon was potentially lower than that of the epipelon, epilithon (shallower water) and phytoplankton as light levels were lowest on sand. In contrast, light limitation was not likely controlling epipelon or epilithon growth because mud substrates and stones occurred in shallow waters (