Variability of light absorption coefficients by different size fractions of suspensions in the southern Baltic Sea

<p>Measurements of light absorption coefficients by particles suspended in seawater (<span class="inline-formula"><i>a</i>_p(<i>λ</i>)</span>), by phytoplankton (<span class="inline-formula"><i>a</i>_ph(<i>λ</i>)</span>) and detritus (<span class="inline-formula"><i>a</i>_d(<i>λ</i>)</span>) were carried out in the southern Baltic Sea for the original seawater samples and four size fractions: pico-particles (0.2–2 <span class="inline-formula">µ</span>m), ultra-particles (2–5 <span class="inline-formula">µ</span>m), nano-particles (5–20 <span class="inline-formula">µ</span>m) and micro-particles (20–200 <span class="inline-formula">µ</span>m). Chlorophyll <span class="inline-formula"><i>a</i></span> (Chl <span class="inline-formula"><i>a</i></span>) and suspended particulate matter (SPM) concentrations were determined. The proportions of particles from the size classes in the <span class="inline-formula"><i>a</i>_p</span>(443), <span class="inline-formula"><i>a</i>_ph</span>(443) and <span class="inline-formula"><i>a</i>_d</span>(443) were determined. Pico- and ultra-particles had the largest contribution to the total particle absorption – an average of 38 % and 31 %. Particles of 5–20 <span class="inline-formula">µ</span>m accounted for approximately 20 % of <span class="inline-formula"><i>a</i>_p</span>(443) and <span class="inline-formula"><i>a</i>_ph</span>(443) and 29 % of <span class="inline-formula"><i>a</i>_d</span>(443). The contribution of particles <span class="inline-formula">&gt;</span> 20 <span class="inline-formula">µ</span>m averaged 5 %–10 %. In total SPM contribution of micro-particles averaged 17 %; nano-, ultra- and pico-particles averaged 29 %, 26 % and 27 %, respectively. In total Chl <span class="inline-formula"><i>a</i></span>, the proportions of pico- and ultra-particles averaged 35 % each, nano-particles 16 % and micro-particles 15 %. Temporal and spatial variability of particles contributions in size classes were observed.</p> <p>The average chlorophyll-specific and mass-specific light absorption coefficients, i.e., light absorption coefficients normalized to Chl <span class="inline-formula"><i>a</i></span> or SPM, were determined for all size fractions. The chlorophyll-specific coefficients <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M21" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi>a</mi><mi mathvariant="normal">p</mi><mrow><mo>(</mo><mi mathvariant="normal">Chl</mi><mspace linebreak="nobreak" width="0.25em"/><mi>a</mi><mo>)</mo></mrow></msubsup><mo>(</mo><mi mathvariant="italic">λ</mi><mo>)</mo></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="47pt" height="19pt" class="svg-formula" dspmath="mathimg" md5hash="40f35f4d95116e46bacae2d9b2f9ca08"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-20-2525-2023-ie00001.svg" width="47pt" height="19pt" src="bg-20-2525-2023-ie00001.png"/></svg:svg></span></span>, <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M22" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi>a</mi><mi mathvariant="normal">d</mi><mrow><mo>(</mo><mi mathvariant="normal">Chl</mi><mspace linebreak="nobreak" width="0.25em"/><mi>a</mi><mo>)</mo></mrow></msubsup><mo>(</mo><mi mathvariant="italic">λ</mi><mo>)</mo></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="47pt" height="19pt" class="svg-formula" dspmath="mathimg" md5hash="484463fc2a801e349f929b08d85ef511"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-20-2525-2023-ie00002.svg" width="47pt" height="19pt" src="bg-20-2525-2023-ie00002.png"/></svg:svg></span></span> and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M23" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi>a</mi><mi mathvariant="normal">ph</mi><mrow><mo>(</mo><mi mathvariant="normal">Chl</mi><mspace width="0.25em" linebreak="nobreak"/><mi>a</mi><mo>)</mo></mrow></msubsup><mo>(</mo><mi mathvariant="italic">λ</mi><mo>)</mo></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="47pt" height="20pt" class="svg-formula" dspmath="mathimg" md5hash="e3667091eb0219f241e71e8b8080ffd1"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-20-2525-2023-ie00003.svg" width="47pt" height="20pt" src="bg-20-2525-2023-ie00003.png"/></svg:svg></span></span>, <span class="inline-formula">±</span> standard deviations, do not allow clear separation of the individual fractions. For mass-specific coefficients, <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M25" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi>a</mi><mi mathvariant="normal">p</mi><mrow><mo>(</mo><mi mathvariant="normal">SPM</mi><mo>)</mo></mrow></msubsup><mo>(</mo><mi mathvariant="italic">λ</mi><mo>)</mo></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="45pt" height="19pt" class="svg-formula" dspmath="mathimg" md5hash="f8acb083a9476b35404aaac78f56bb61"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-20-2525-2023-ie00004.svg" width="45pt" height="19pt" src="bg-20-2525-2023-ie00004.png"/></svg:svg></span></span>, <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M26" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi>a</mi><mi mathvariant="normal">d</mi><mrow><mo>(</mo><mi mathvariant="normal">SPM</mi><mo>)</mo></mrow></msubsup><mo>(</mo><mi mathvariant="italic">λ</mi><mo>)</mo></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="45pt" height="19pt" class="svg-formula" dspmath="mathimg" md5hash="383605d95c5b0772ef10463a50702283"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-20-2525-2023-ie00005.svg" width="45pt" height="19pt" src="bg-20-2525-2023-ie00005.png"/></svg:svg></span></span> and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M27" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi>a</mi><mi mathvariant="normal">ph</mi><mrow><mo>(</mo><mi mathvariant="normal">SPM</mi><mo>)</mo></mrow></msubsup><mo>(</mo><mi mathvariant="italic">λ</mi><mo>)</mo></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="45pt" height="20pt" class="svg-formula" dspmath="mathimg" md5hash="db41c69c21e2757016982821893ccb5a"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-20-2525-2023-ie00006.svg" width="45pt" height="20pt" src="bg-20-2525-2023-ie00006.png"/></svg:svg></span></span>, it is possible to distinguish between large particles (20–200 <span class="inline-formula">µ</span>m) and small and medium particles (0.2–20 <span class="inline-formula">µ</span>m). These results will allow monitoring of suspended matter in size classes in optically complex waters of southern Baltic Sea.</p>