Abstract
In this study we have compiled a long-term monitoring dataset from the inner Oslofjorden and supplemented it with short-term research data from the same station. Using generalized additive models analysing the data from this time series, we have examined how chlorophyll-a, hydrography, and various nutrient concentrations have changed during 1973–2017 and how they correlate. We describe the seasonality in chlorophyll-a, nitrogen, phosphorus, Secchi-depth, temperature and salinity and how the levels of each variable have changed the last forty years. The results show specifically how levels of chlorophyll-a have decreased significantly and how this correlates with decrease in nitrogen and phosphorus levels. Our results show a significantly positive correlation between chlorophyll-a and phosphorus during spring bloom and between chlorophyll-a and nitrogen during autumn bloom. However, phosphorus levels have increased again during the last 20 years, but chlorophyll-a levels are still low, indicating that the chlorophyll-a level currently may be controlled by the continuous decreasing trend in nitrogen. If nitrogen increase again, the chlorophyll-a level may also begin to increase. The impact of increasing temperature and possible change in starting point for the growing season should be studied further.
Highlights
Eutrophication is a major threat to coastal marine ecosystems worldwide, and has been known as such for more than 50 years (Ryther and Dunstan, 1971)
In the inner Oslofjorden, as in other Norwegian fjords, it is common to have a second bloom in May or June when snow is melting in the highlands and nutrient-rich melting water is reaching the coast (Paasche and Erga, 1988)
Our analyses revealed that chlorophyll-a levels have been significantly reduced during the period 1980 to 1990 and that this reduction is most likely due to a major reduction in the supply of total nitrogen and phosphorus
Summary
Eutrophication is a major threat to coastal marine ecosystems worldwide, and has been known as such for more than 50 years (Ryther and Dunstan, 1971). Increased supply of nutrients to the ecosystem causes increased phytoplankton production and can lead to reduced surface water quality (Nixon, 1995; Paasche and Erga, 1987). Spring bloom starts when increased daily irradiance and increased stratifica tion cause phytoplankton to grow rapidly in the upper mixed layer on nutrients supplied by prior turbulence and convective mixing of the upper water column (Erga and Heimdal, 1984; Kristiansen et al, 2001; Sverdrup, 1953). The autumn bloom occurs when and if seasonally increasing vertical mixing (con vective cooling and winds) renews the nutrient supply in the euphotic zone before light availability becomes limiting (Findlay et al, 2006; Paasche and Ostergren, 1980)
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