The current TP values were two to six times higher than the desired target values for the German inner CWB of the Baltic Sea during summer. DIP concentrations were at 5 out of 8 stations in a determinable range (>0.05 µmol L
−1), but DIP was always low (on median ≤0.1 µmol L
−1) at stations with highest cyanobacteria proportion (DB2, DB16, RB10). However, elevated DIP concentrations were frequently observed during summer at all stations. The chl
a values found in this work were two to twelve times higher at the same TP levels compared to e.g. Danish coastal waters (Kronvang et al.
2005) or brackish lagoon systems in Australia (Cook and Holland
2012). Chl
a values in those studies were probably lower, because of elevated dilution with open sea water as indicated by salinity. The DIP and TP values observed in this study indicate that P might only be the limiting factor at the observed stations during winter and spring. The high DIN values promote the formation of high phytoplankton blooms during spring, as indicated by the positive correlation of Bacillariophyceae, as spring-dominant phytoplankton, with TN and overall decreasing nutrient concentrations. The TN:TP as well as the DIN:DIP ratio during spring point to a P limitation. The high DIN:DIP ratios are in contrast to those of the Baltic Proper, where even during winter times DIN:DIP ratios of 16:1 are measured with low DIN concentrations (~ 8 µmol L
−1 DIN) (Walve and Larsson
2007). The most important observation during seasonal development of phytoplankton is the fact that DIN becomes unavailable in summer whereas DIP still occurs, which might explain the dominance of cyanobacteria. A N-limitation was also described, for example, for most days in Danish coastal waters (Kronvang et al.
2005), Australian lagoons (Cook and Holland
2012) and the Thames estuary (Nedwell et al.
2002) during decade-long water quality surveys. Consequently, the high chl
a concentrations are mainly controlled by N rather than by P, even though the TN:TP ratios indicate P limitation. A cause for that can be the cyanobacteria dominance. Finkel et al. (
2010) showed in their review that cyanobacteria can have higher N:P ratios (up to 50:1) compared to other taxa. These findings indicate that the TN:TP ratio found in this work does not necessarily indicate a P limitation. Therefore, it is so important to evaluate carefully the phytoplankton composition. Albrecht et al. (
2017) showed in their most recent work that the pure morphological identification of cyanobacteria as used by environmental agencies is not possible. These authors determined, for example,
Aphanothece and
Synechococcus morphologically at the inner coastal water body stations DB2, DB16 and RB10. However, the genetic determination contained mainly members of the
Cyanobium clade, which may be misidentified morphologically due to its great morphological plasticity.
Cyanobium is currently described to lack N-fixation genes (Scanlan et al.
2009). On the other hand, the N-fixation genes are polyphyletic and show a patchy distribution in the cyanobacteria phylogeny, i.e. not only the heterocystous species, but also many non-heterocystous taxa possess these genes (Latysheva et al.
2012). Therefore, it is at present impossible to evaluate whether N-limitation, as indicated by DIN:DIP, favours N-fixing cyanobacterial species, and whether there is a limitation at all. The concentrations of easily bioavailable dissolved organic N and P have to be considered to address this question. However, both parameters are currently not monitored.
The discrepancy between phytoplankton biomass and nutrients can be attributed to resilience mechanisms. The adaptation capabilities of cyanobacteria to low P levels need to be discussed since they were the dominating group at some inner coastal waters. Cyanobacteria are usually very small (<10 µm) and thus have a high cell surface-to-volume ratio. This ratio supports small-sized cells in term of competitive nutrient uptake (e.g. Friebele et al.
1978). Cyanobacteria are also capable to luxury consume P and store it as polyphosphate (e.g. Ritchie et al.
2001). This luxury consumption can explain the discrepancy of high TP values at moderate chl
a concentrations, but not vice versa. Additionally, they are capable to cause higher turbidity per unit P and even adapt to the self-induced light limitation suppressing other phytoplankton species (Scheffer et al.
1994), which would explain high chl
a concentrations at lower TP concentrations found in this study. These authors characterised the described data as hysteresis mechanism on the dominance of cyanobacteria in shallow lakes, which is in agreement with the results in the present study. Stations (i.e. DB2, DB16) with 75–90% cyanobacteria dominance had the lowest Secchi depth at same (DB16) or even half (DB2) the chl
a concentration compared to other analysed stations (i.e. KHM, P42, see Table
3). The described resilience was already postulated by Sas (
1989) for lakes. The luxury P uptake and intracellular P accumulation is especially important in dynamic aquatic systems like CWB (Aubriot et al.
2011), where nutrient supply typically occurs as seasonally fluctuating pulses. The adaptations for light limitation would explain the high chl
a concentrations found in this work, but not the DIP concentrations that should have been luxury consumed. These findings lead to the conclusion that phytoplankton might be P saturated and shows a severe N-limitation during summer.
The described increased phytoplankton growth during high precipitation events might be explained by elevated nutrient input through the catchment area. This work exhibited that during years with above-average precipitation the water bodies were affected seasonally. An effect on phytoplankton was often measured, although the data set revealed no consistently higher TP concentration during those rainy summer months (vegetation periods). An elevated nutrient input during storm events (Pinckney et al.
2001) and humid years (Jordan et al.
2003) was described for other estuaries and even for tile drains within the catchment areas of German coastal zones (Zimmer et al.
2016). However, not only bioavailable P but also N is transported into coastal systems during extreme weather events and elevating the DIN:DIP ratio (up to 60:1 during storm events) (Rees et al.
2009). The elevated chl
a concentrations during extreme years could be rather explained by the impact of N than by P. This assumption could also explain why higher DIP concentrations occurred during years with average precipitation, compared to years with higher precipitation. The enhanced demand for DIP during growth with simultaneous DIN loading may explain the low DIP concentrations during extreme years. All these findings agree with Howarth and Marino (
2006) who postulated in their review that N is the eutrophication causing agent in most coastal marine systems, but primary production can only be reduced with a coupled N- and P-reduction.