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DOC and POC in the water column of the southern Baltic. Part I, Evaluation of factors influencing sources, distribution and concentration dynamics of organic matter

Maciejewska A., Pempkowiak J.

Oceanologia

2014

No. 56 (3)

523--548

Organic substances are important components of the marine environment as they determine the properties of seawater and the key biogeochemical processes taking place in it. Organic carbon (OC) is a measure of organic matter. For practical purposes, OC is divided into dissolved organic carbon (DOC) and particulate organic carbon (POC). Both DOC and POC play a major role in the carbon cycle, especially in shelf seas like the Baltic, where their concentrations are substantial. In a three-year study (2009-2011) seawater samples for DOC and POC measurements were collected from stations located in the Gdańsk Deep, the Gotland Deep and the Bornholm Deep. The accuracy and precision of analysis were satisfactory; the recovery was better than 95%, and the relative standard deviation was 4% (n = 5). Concentrations of chlorophyll a, phaeopigment a, salinity, pH and temperature were also measured in the same samples. These parameters were selected as proxies of processes contributing to DOC and POC abundance. The aim of the study was to address questions regarding the vertical, horizontal and seasonal dynamics of both DOC and POC in the Baltic Sea and the factors influencing carbon concentrations. In general, the highest concentrations of both DOC and POC were recorded in the surface water layer (DOC ~4.7 mg dm-3, POC ~0.6 mg dm-3) as a consequence of intensive phytoplankton activity, and in the halocline layer (DOC ~5.1 mg dm-3, POC ~0.4 mg dm-3). The lowest DOC and POC concentrations were measured in the sub-halocline water layer, where the values did not exceed 3.5 mg dm-3 (DOC) and 0.1 mg dm-3 (POC). Seasonally, the highest DOC and POC concentrations were measured during the growing season: surface DOC ~5.0 mg dm-3; sub-halocline DOC ~4.1 mg dm-3 and surface POC ~0.9 mg dm-3, sub-halocline POC ~0.2 mg dm-3. The ANOVA Kruskal-Wallis test results indicate statistically significant differences among the three study sites regarding average concentrations, and concentrations in particular water layers and seasons. It shows that concentrations of DOC and POC differ in sub-basins of the Baltic Sea. The differences were attributed to the varying distances from river mouths to study sites or the different starting times and/or durations of the spring algal blooms. Statistically significant dependences were found between both DOC and POC concentrations and Chl a (phytoplankton biomass), pH (phytoplankton photosynthetic rate), pheo (zooplankton sloppy feeding), salinity (river run-off and North Sea water inflows) and water temperature (season). This was taken as proof that these factors influence DOC and POC in the study areas.

Problematyka wyznaczania przepływu nienaruszalnego w warunkach małych zlewni nizinnych

Szymczak T.

Woda-Środowisko-Obszary Wiejskie

2002

T. 2, z. 1

137--154

W artykule opisano metody określania wartości przepływów nienaruszalnych według kryteriów: hydrobiologicznego, ochrony przyrody, rybacko-wędkarskiego, sportu i turystyki wodnej. Rozwinięto zagadnienie obliczania przepływu nienaruszalnego według kryterium hydrobiologicznego w odniesieniu do małych cieków nizinnych. Oceniono możliwość stosowania metody opartej na zakładanym związku wartości tego przepływu z przepływem średnim niskim SNQ w danym profilu oraz minimalną niezamulającą prędkością ruchu wody zwaną prędkością miarodajną. Wykazano, że w przypadku małych cieków nizinnych wartości przepływów nienaruszalnych wyznaczone metodą opartą na związku między natężeniem przepływu a prędkością miarodajną są znacznie wyższe (od 1,8 do 28,2 razy) od przepływu średniego niskiego SNQ. Obliczany w ten sposób przepływ nienaruszalny obarczony jest błędem, przekraczającym w niektórych wypadkach 70%. Błąd spowodowany jest dużą zmiennością w czasie warunków hydraulicznych przepływu w korytach małych cieków nizinnych. Przepływy nienaruszalne obliczane zarówno na podstawie związku z prędkością miarodajną jak i z wielkością przepływu średniego niskiego mają przypadkowe wartości, które nie znajdują uzasadnienia z punktu widzenia niezbędnego minimum potrzebnego do zachowania życia organizmów wodnych. W tej sytuacji najlepszym rozwiązaniem jest indywidualne traktowanie każdego cieku i określanie przepływu nienaruszalnego w oparciu o parametry hydrauliczne koryta z uwzględnieniem ich sezonowej zmienności. Wymaga to opracowania odrębnej metodyki obliczeń przepływów nienaruszalnych. Stwierdzono znaczną niestabilność wartości przepływu nienaruszalnego wyznaczanego na podstawie przepływu średniego niskiego. Wynika ona z zależności przepływu SNQ od przyjętego do obliczeń okresu obserwacyjnego, nawet w przypadku dwudziestoletnich ciągów danych.

The author describes the methods of estimating minimum acceptable flow according to the following criteria: hydrobiology, environmental protection, fish culture, angling, water sports and tourism. The assessment of the minimum acceptable flow values for small lowland catchments based on the hydrobiological criterion, is investigated in details. The metod based on the assumed relation between the minimum acceptable flow and the average low flow SNQ on the one hand, and the minimum non silt up flow for a specified river crosssection on the other, is evaluated. The investigation shows the frequency of flow with intensities below the minimum acceptable flow calculated according to the hydrobiological criterion and the variation of the average low flow for different calculation periods. The study is based on hydrometric measurements and systematic hydrological observations carried out between 1966 and 1995 in five small lowland catchments of areas varying from 32 to 82 km². Additionally, daily observations of the Narew river at Suraż cross-section (3376 km²) are included. The minimum acceptable flow, calculated with the use of the flow intensity – the minimum non silt up flow relation method, was considerably higher (from 1.8 to 28.2 times) than the average low flow SNQ. The minimum acceptable flow intensities error exceeded 70 % in some cases. The error depended mainly on the variation of the hydraulic flow conditions within the small lowland stream channels. It can be assumed, that for the small lowland rivers, the minimum acceptable flow equals the minimum non silt up flow. In such a case, the time of water consumption can be considerably elongated, but at the same time the flow intensities correspond to flow velocities Lower than velocities assumed for the minimum non silt up flow. It was also pointed out, that the minimum acceptable flow calculations based on the relation between the minimum non silt up flow velocity and the average low flow in the case of small lowland rivers, are of random values. Thus, they are useless for water wildlife preservation. The best solution seems to be the calculation of the minimum acceptable flow for a stream channel, taking into account seasonal changes and the specific hydraulic parameters. This requires developing a new methodology of the minimum acceptable flow calculations. In the case of the Narew river, whose catchment has considerably larger area, the minimum acceptable flow, calculated with the parametrical method as 50 % of the average low flow, describes conditions with no shortage of water during a year and during longer periods of time. The investigation also revealed a strong relationship between the average low flow values and the period of time taken for calculations. As a result, the values of the calculated minimum acceptable flow were unstable.