## 1 Introduction  
Trace metals in natural waters are distributed among various physical phases: particulate, colloidal and truly dissolved [1]. Dissolved and particulate metals are operationally separated with a 0.45 or 0.2 \(\upmu\)m pore size filter, and the colloidal fraction is defined as having at least one dimension in the size range between 1 nm in diameter (which roughly equates with molecules of 1 kDa nominal molecular weight) and 1 \(\upmu\)m [2, 3, 4]. The interactions among these phases appear to govern the fate and transport of the metals in the environment [5]. In coastal waters, especially in estuaries, processes affecting the trace metal speciation and fractionation play a key role in their transport to the open sea. Due to their large specific area, exposing a high number of reactive functional groups, colloidal particles play an important role in trace metal scavenging and thus control their cycling in coastal waters and their downward transport in the water column [5, 6]. The estuarine reactivity of colloidal trace metals appears to be governed by their affinity for organic matter and for particulate surfaces [5]. Concentration and type of organic matter have a great influence on the aggregate formation and mobility of many trace metals (e.g., Zn, Pb, Cd, Cu, Ni and Co) in the estuaries. Due to their affinity to trace metals, organic ligands can control their size partitioning between solution and suspended particles and become the dominant carrier phase for these metals [7, 8, 9, 10]. Besides organic matter, Mn- and Fe-rich colloids can act as the principal sorptive carrier phase for some trace elements in estuarine waters (e.g., As, Pb and Zn) [7, 9]. Colloids play an important role in the exchange processes of metals between the sediment and the water column [11]. The bridging of inorganic colloids by organic substances (humic substances and biopolymers) in an estuarine mixing area represents the major route for the aggregate formation and elimination of trace metals from the water column by sedimentation [3; 12]. On the other hand, for some metals, distribution can be governed by the release from colloidal aggregates due to the competitive effects of major cations/anions (e.g., the formation of stable Cd chloro-complexes) [13; 14; 15]. While the aggregation is the main process causing the trace metal removal from the water column, the release of metals from particles during the sediment resuspension is the main counteractive process in the freshwater-seawater mixing area [16; 17]. Additional various biotic and abiotic processes (e.g., metal bioptake, production of biogenic ligands, photodegradation or microbial degradation of organic matter, biological impact on metal deposition and immobilization from sediments) are of great importance for trace metal distribution and colloid formation/degradation within the estuary [16]. Most of the studies that were conducted in estuarine systems dealt with the particulate and dissolved fractions. However, separation of the dissolved from the particulate phase does not isolate colloids but rather includes them in the dissolved fraction. Colloids can be isolated by a variety of physicochemical techniques such as ultrafiltration, dialysis, gel techniques, field flow fractionation and capillary electrophoresis [6; 18; 19; 20; 21; 22]. Among them, a low-cost and easily implemented technique, centrifugal ultrafiltration, is considered to be a high-efficiency colloidal substance separation technology [23; 24]. The KRA River estuary is a very interesting system for the investigation of trace metal partitioning, since for most of the year it features a pristine environment with low organic content. Namely, due to the scarce vegetation, the absence of significant anthropogenic influence and self-purifying ability of the KRA River, the estuary, as well as the coastal sea, are unusually clean in terms of trace elements, terrigenous material and biomarkers [8; 25; 26; 27; 28; 29; 30].