Aarhus University Seal / Aarhus Universitets segl

No. 166: Lakes 2014

Johansson, L.S., Søndergaard, M., Nielsen, A., Landkildehus, F., Kjeldgaard, A., Sortkjær, L. Windolf, J. & Bøgestrand, J. 2015. Søer 2014. NOVANA. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 84 s. - Videnskabelig rapport fra DCE - Nationalt Center for Miljø og Energi nr. 166. http://dce2.au.dk/pub/SR166.pdf

Summary

In 2014, 48 lakes were included in the control monitoring programme – 41 freshwater lakes and seven brackish lakes. Eighteen of the lakes were covered by the monitoring of lake development over time, while 30 were subject to the monitoring of current environmental state. The monitoring programme covers a wide span of lakes with regard to, for instance, size and depth and range from clean to heavily polluted lakes as a result of existing or previous wastewater discharge. In all, during 2011-2014, 120 lakes covered by the programme for monitoring of environmental state have been examined.

The Danish Nature Agency’s decentralised units are responsible for the standardised sample collection. All data collected are reported to the National Topic Centre for Freshwater, which prepares annual progress reports on the general state and development of Danish lakes.

This year’s report includes results for trends observed in the period 1989-2014 in 15 of the 18 lakes included in the monitoring of development over time. For the remaining three lakes, sufficiently long time series of data do not exist to allow their inclusion in a description of the development; these lakes are therefore treated individually. In addition, a short status is given of the current state and trends in the 120 lakes subject to monitoring of environmental state for the period 2011- 2014. Finally, a brief status is provided of the environmental state in 357 lakes encompassed by the operational programme, which were investigated during 2011-2014.

A comprehensive overview of the environmental state of the investigated monitoring lakes for four key parameters is given in table 1.1 in the report. The environmental state of the operationally monitored lakes (lakes that are not expected to meet the set objectives) is generally poorer than in the lakes included in the control monitoring.

Control monitoring of development

For all four key parameters in the 15 lakes included in the control monitoring of lake development, improvements have been recorded from the period 1989-1994 to 2014 (table 1.2 in the report). Regarding average values, the most pronounced improvement is noted for total phosphorus, which decreased by 57%, and the smallest improvement for Secchi depth, which increased by 22%. The improvement has been greater for average than for median values due to the fact that the improvement was greatest in the most nutrient-rich lakes.

The most significant decline occurred in the first part of the monitoring period. Of the 15 lakes there has been a significant decrease in summer total phosphorus concentrations in 12 over the entire monitoring period 1989-2014. For the past 10 years, this is the case in only five of the lakes (table 1.3 in the report).

Also the nitrogen concentration has decreased markedly since initiation of monitoring, and in the period 1989-2014 a significant decrease occurred in 14 of the 15 lakes. In the last 10 years, there has been a significant decrease in seven of the lakes; thus, in some of the lakes the reduction in nutrient levels, otherwise occurring primarily in the first part of the monitoring period, continues.

From 1989-2014, the concentration of chlorophyll a has declined in seven of the 15 lakes and Secchi depth has increased in nine lakes (table 1.3 in the report). In three lakes, however, enhanced concentrations of chlorophyll a have been registered, and in one lake decreased Secchi depth has been observed. In the past 10 years, the concentrations of chlorophyll a and Secchi depth have not changed in most lakes.

The annual relative change in summer averages of the four key parameters since 1989 is shown as an index in figure 1.1 in the report where the values in 1989 are set to 1 for all four parameters. The general picture is that of unchanged or slightly deteriorating conditions from 1989 to 1993, followed by a substantial improvement until 1996 after which there has been a steady improvement in nutrient levels and chlorophyll a. As also shown in table 1.2 in the report, the improvement in Secchi depth is less marked and during the past 10 years there has been no development trend.

The annual development in concentrations of total phosphorus, total nitrogen, chlorophyll a and Secchi depth in the 15 lakes since 1989 is also illustrated in figure 1.2 in the report. The figure shows the three quartiles (25%, median and 75%) and partly demonstrates that the greatest changes occurred in the beginning of the monitoring period and, second, that the most pronounced changes are found in the most nutrient-rich lakes.

For phosphorus and nitrogen, there has been a marked reduction in the upper 75% fractile throughout the period, while the median value has decreased slightly, and the lower 25% fractile is largely unchanged. Chlorophyll a declined significantly during the first 10 years in the most chlorophyll-rich lakes, but during the last 15 years the level has remained relatively stable. The trend in median values and in the lower 25% fractile has been modest. Secchi depth has increased throughout the period for all three fractiles, but – again – primarily in the first 10 years.

For 10 of the intensively monitored lakes with well-defined water balances, mass balances have been calculated and source apportionment has been undertaken. Generally, both the concentrations of nitrogen and phosphorus running to and from the lakes have decreased significantly in most of the lakes, for phosphorus especially in the first part of the monitoring period as a result of reduced loads from wastewater treatment plants. Therefore, the concentration of phosphorus in the water that flows to and from the lakes has not shown a similar downward trend within the past 10 years. By contrast, for nitrogen, a persistent downward trend in concentrations has generally occurred in the water flowing to and from the lakes during the monitoring period. Thus, the average concentrations of phosphorus and nitrogen in the water supply to the lake have decreased by, respectively, 22 and 22% between the periods 1990-94 and 1995-2000, while the decrease is 8-10 and 13-15%, respectively, between the periods 1995-2000 to 2001-2006 and 2001-2006 to 2007-2013. Also absolute nitrogen and phosphorus inputs to the lakes have decreased. In contrast, relative retention exhibits large fluctuations and generally no significant change in the monitoring periods. Over the entire monitoring period, retention has, however, increased in four and five lakes for phosphorus and nitrogen, respectively.

Control monitoring of environmental state

The 120 lakes included during 2011-2014 in the control monitoring of environmental state represented seven lake types – 17 brackish lakes and 10 low-alkaline lakes, one of which is deep. The majority of the lakes are fresh, alkaline and shallow (66) or deep (27). The overall condition of all the lakes is depicted in table 4.1.

The 66 shallow, alkaline lakes are generally nutrient-rich with a summer median of 0.09 mg P/l, a Secchi depth of 0.9 m and  a chlorophyll a concentration of 42 µg/l. Macrophyte coverage is relatively low (median 8.1%) and the fish stock generally dominated by cyprinids (roach, bream and rudd). Compared with the period 2004-2009, during 2011-2014 there has been a significant decrease in the concentration of total nitrogen and a significant increase in the number of macrophyte species.

Nutrient levels in the 27 deep, alkaline lakes are generally slightly lower than in the shallow lakes, with a summer median for total phosphorus of 0.06 mg P/l, a Secchi depth of 2.0 m and a chlorophyll a concentration of 18 µg/l. The median fish density is lower than in shallow lakes, but also here dominated by cyprinids; generally with a higher proportion of predatory fish than in the shallow lakes, however. Compared with the period 2004-2009, there has been a significant decrease in the concentrations of total phosphorus and total nitrogen. Other positive signs include a significant increase in the number of submerged macrophyte species and in the percentage of predatory fish, and there is also a trend towards an increase in the depth limit of the submerged macrophytes.

Operational monitoring

This year, a brief status is given of the environmental state in the 357 lakes included in the operational programme, which were examined in the period 2011-2014. The lakes represent a total of nine types and a general overview of the environmental state is given for the most frequent types. The lakes in the operational programme are not representative of Danish lakes in general as the lakes are not randomly chosen, but selected on the basis of the assumption that they are deemed at risk of failing to meet the  goals for nature and environment in 2015 (European Union 2000). The lakes are not necessarily a representative sample of all lakes that are scheduled to be studied within the framework of the operational programme.

Overall, the lakes in the operational programme are nutrient rich and the environmental state is slightly poorer than that observed in the lakes included in the control monitoring of environmental state (table 4.1). The most common lake type is shallow, alkaline, non-brown-water lakes (146 lakes), which are generally nutrient rich, but also range widely from nutrient-poor lakes to lakes in very poor condition. Nutrient levels in their brown-water counterparts (49 lakes, including one deep lake) are at least just as high, though with relatively more lakes having a significant macrophyte-covered area. This may be because these lakes are generally shallower than the non-brown-water lakes.

The 61 deep, alkaline non-brown-water lakes included in the operational monitoring are more nutrient poor than the shallow lakes, and their environmental state is similar to that observed in the deep, alkaline lakes included in the control monitoring of environmental state.

The most nutrient-poor lakes are the low-alkaline, non-brown-water lakes (in total nine lakes), part of which are water-filled, highly nutrient-poor, abandoned lignite mines.

Brown-water, low-alkaline lakes (21 lakes, all shallow) are more nutrient rich with high concentrations of chlorophyll a and low Secchi depth (50 cm).

Brackish lakes (71 lakes) are generally the most nutrient-rich lakes in the programme, especially the brown-water lakes (22) with a summer median for total phosphorus of 0.43 mg/l.