Over the past few years the benthic boundary layer has been studied intensively, as chemical gradients are most obvious and microbial activities are maximal in this layer supporting biofilm growth (Beulker & Gunkel, 1996; Hiscock & Gruschek 2002). The biofilm is a complex community of algae, bacteria, fungi and invertebrates, living in the interstice system: bacteria and algae produce extra-cellular polymeric substances (EPS), forming a dense three-dimensional structure (Lawrence et al., 1998, 2002; Paterson, 2001). The development of a high structured biofilm, consisting of several groups of organisms occurs only under oxic conditions; the limiting oxygen concentration is not yet knows. Do to the occurrence of algae, a daily cycle with oxygen enrichment during day and oxygen depletion during night must be considered, thus critical oxygen concentrations are reached only at night. Thus, the development of a dense layer of superficial filamentosus algae like the schmutzdecke in GWR ponds will in general impact the development of a high diversity biofilm because of light limitation in the interstice and, as a consequence, decrease of the interstitial algae as well as by an increased probability of anoxic conditions during night (oxygen depletion due to respiration of the schmutzdecke’s algae). Concerning oxygen balance, two different states have to be clearly distinguished, anoxic or anaerobic infiltration water and the formation of a micro-zoning in the pore system with small anoxic and anaerobic zones. Microscopic analyses point out, that too under oxic conditions a micro- zoning occurs with some anaerobic areas maybe in dead end pores, verified by the presence of iron sulphide (pyrite).
Biofilms and the associated algae are of special importance for water purification processes due to a net oxygen production as well as an adsorption of ions like toxic metals and of dissolved organic matter (DOM). The bacterial community consume and metabolize DOC, and the excretion of exoenzymes leads to an elevated mineralization efficiency of DOM and some inorganic polymers (e. g. polyphosphate, colloids; Decho, 2000; Wingender & Flemming, 2001). Other well-known properties of biofilms are the high water binding capacity and the stabilization of the surrounding sediment grains (Yallop et al., 2000). The biofilm has a three dimensional structure and is the basis for a micro-zoning of the filter area (Flemming et al., 1999), dead end pores occur and serve in retention of small particles (Auset & Keller, 2006), but too enable the formation of oxygen mirco-zoning in the interstice; some degradation processes, e. g. of drugs, are linked to a small scale change of oxidative and reductive conditions in the filter.
The most important factor for the spatial build up and the function of the biofilm is the excretion of EPS by bacteria and algae as ‘housing’. Structural determinations of EPS forming biofilms have shown substantial progresses during the last decades, and neutral and polyanionic polysaccharides as well as peptids (glycolproteins, lipoproteins) form EPS (Characklis et al., 1990; Wingender et al., 1999; Flemming & Wingender, 2001; Sutherland, 2001). The EPS is a secretion of algae (e. g. diatoms excrete mucus for movement or cell sheaths) or part of the cell structure (e. g. lipopolysaccharides of gram negative bacteria). EPS from hydro-gel to viscose elastic structures like filaments, nets and plaques (Figure 10). A conversion of EPS, that means a re-construction due to enzymatic degradation by hydrolase and condensation of the formed oligosaccharides can occur, but in general the stability of the
EPS is very high, and EPS structure consist over some weeks to months, which mean the EPS outlive the builders for a long-time.
The biofilm biocoenosis is built up by bacteria, fungi and algae, whereas the vertical distribution of the algae is limited by the transparency, which means the light penetration into the interstice. Bacteria and fungi settle too in deeper, aphotic zones; bacteria build up an adapted community, on one hand related to the DOC input and its degradability and on the other hand by the micro-zoning in the three dimensional interstice system. It must be assumed that the bacterial community is to a high degree a local one, but up to now only few investigations are available (Kolehmainen et al., 2007). The abundance of bacteria at Lake Tegel, Berlin, Germany, was very high, and they reached up to 2 x 109 cells gram-1 sediment, determined with DAPI8 fluorescence technique; the vertical distribution showed highest cell numbers at the surface layer of 0 – 5 cm, while in depth of > 20 cm, still 0.2 x 109 cells gram-1 sediment were found. Further specification of these bacteria can be done using FISH9 or PCR10 (Spring et al., 2000, Emtiazi et al., 2004).
The bacterial community in the interstice seems to be a complex system of linked species, adapted to DOM characteristics, fixed in the three dimensional EPS structure and being at least partly located in dead end pores, this give a analogous community to the bacterial flocs in surface water (Zimmermann-Timm, 2002).
Figure 10. Biofilm with some algae and bacteria cells and a dense fibrillose net structure of
extracellular polymeric substances (EPS) in the interstice of the water-sand-boundary layer at the bank filtration site at Lake Tegel, Berlin, Germany (depth of 3 – 4 cm, 17.05.2004).
8 4'6-diamidino-2-phenylindole-2HCl, a fluorescent dye for DNA. 9 Fluorescence in situ hybridisation.
Beside bacteria cells activity, the bacterial exo-enzyme activity (e.g. aminopeptidase, glucosidase, phosphatise) is of high significance for degradation processes of DOM, too (Miettinen et al., 1996, Hendel et al., 2001).
The investigations carried out in the fine sand infiltration site of Lake Tegel, Berlin, Germany, offered a high portion of POM with a maximum at the surface layer with 15 mg g-1 sediment, stretching down to 50 cm (Figure 11; Gunkel et al. 2008). Epipsammic algae11 occur with a high biomass in the upper interstitial zone of about 0 – 6 cm depth, and it has to be pointed out that planktonic algae species from the lake water are only transported into the interstice to a small extent (Beulker & Gunkel, 1996; Gunkel et al., 2008). Thus this water- sediment-boundary layer serves as a mechanical filter for these algae cells – a process being of very high importance for the behaviour and fate of all types of POM in bank filtration. This leads to
1. the surficial accumulation of algae cells,
2. an easy resuspension of surficial deposited algae,
3. an insignificant penetration of algae cells into the interstice,
4. a high attraction of the surficial sediment layer for herbivorous interstitial fauna with its burial activity,
oxygen concentrations of deeper sediment layers are not reduced by POM mineralisation but only by DOM concentration and mineralisation.
Concerning toxic cyanobacteria, too, an accumulation at the surficial sediment layer must be expected (see Chapter 6.6).
The occurrence of interstice algae lead to a natural bioproduction in the small photic surface layer of a few centimetres, and both, POC as well as DOC of the infiltrating water is influenced by the in situ production of POC and DOC (Hoffmann & Gunkel 2009b). The vertical distribution of chlorophyll (Chl a) confirmed that interstitial algae biomass forms a significant part of the total POM in the upper interstitial zone of 0 – 6 cm (Figure 12), and primary production is assumed to be the most important source of organic carbon in the interstice. Up to now only biomass and no turnover data of interstitial algae are available, too a lack of information exists concerning pico-algae12 (Dittrich et al., 2004).
At Lake Tegel the Chl a concentrations in the upper 5 centimetres of sediment were very high (21 to 28 µg cm-3) and decreased with depth, only traces of Chl a were detected below 5 cm depth. A Chl a concentration of about 25 µg cm-3 must be evaluated as extremely high compared with the lake water, which even under eutrophic conditions contains only about 20 µg L-1 Chl a. Thus, the total algal biomass in the upper sandy layer of the interstice was about 1000 times higher than in the corresponding water body of Lake Tegel (Gunkel et al., 2008).
11 Algae attached to sand grains.
Figure 11. Depth distribution of FPOM (< 1000 µm) in the interstice of the sandy lake sediments at the bank filtration site at Lake Tegel, Berlin, Germany (means + SD, n = 6, 2004). From Gunkel et al. (2008).
Figure 12. Depth distribution of chlorophyll-a concentrations in the interstice of the sandy lake sediments at the bank filtration site at Lake Tegel, Berlin, Germany (means + SD, n = 1, March to August 2004). From Gunkel et al. (2008).
The origin of these algae were epipsammic13 species, mostly diatoms such as Fragilaria spp., Achnanthes spp., Cocconeis spp., Amphora pediculus, Cymbella spp., Gomphonema spp., Rhoicosphenia abbreviata and Cymatopleura spp.; some other algae classes such as Chlorophyceae, Cryptophyceae and Cyanobacteria occurred (Gunkel et al., 2008). Of high interest is the low significance of planktonic14 algae of the lake water, only three times during the investigation period of one year a significant portion of planktonic algae was observed within the interstice with < 24 % of the total algae abundance; in general the relative abundance of the planktonic algae within the interstice was < 3 % (Gunkel et al., 2008).
In the interstice a food web is build up due to the presence of the meiofauna, small invertebrates living in the interstice, being bacteria, algae or detritus feeder, some of them are carnivorous, too (Wotton et al., 1996; Beulker et al., 1996; Gibert et al., 1998). Fishes like carp serve as predator to the meiofauna. Thus a cycling of organic carbon is build up within the infiltration stretch, and DOC is transferred to POC, but the total amount of organic carbon is reduced due to respiration of the organisms, which means a transformation of organic carbon to CO2. A first analysis of turnover processes point out the high significance of the
interstitial food web (Hoffmann & Gunkel, 2009b).
The abundance of the meiofauna in an undisturbed bank filtration site is very high: In Lake Tegel 19,600 ind. dm-2 sediment area were found as maximum, the annual mean was 8,300 ind. dm-2; this meiofauna abundance corresponds to that of some other Berlin lake littoral zones with a maximum of >20.000 ind. and a mean of 4,400 ind. dm-2 during a three years study (Beulker & Gunkel, 1996). The vertical distribution of the meiofauna showed two different zones, one with very high abundances in 0 – 10 cm depth and the other with a significant decrease in abundance and diversity in depths > 15 cm (< 200 ind. dm-3 in 15 – 20 cm). Thus the abundance of the meiofauna amounts up to 5,000 – 15,000 ind. L-1 sediment – an animal density never reached in surface water.
The meiofauna consists of many different species belonging to different classes and orders of fauna such as flatworms (Tubellaria), rotifera (Rotatoria), roundworms (Nematoda), snails (Gastropoda), mussels (Bivalvia), worms (Annelida), water bears (Tardigrada), seed shrimps (Ostracoda), water flees (Cladocera), copepods (Copepoda) and insect larvae (Chironomidae, Nematocera), a detailed description of the meiofauna of Lake Tegel is given in Beulker & Gunkel (1996).