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<p>Both marine dinoflagellate cysts and terrestrially derived pollen and spores are abundant in coastal sediments close to river mouths, making sediment records from such settings ideal to simultaneously study land–ocean climate interactions, marine productivity patterns and freshwater input over time. However, few studies consider the combined calibration of these palynological proxies in modern coastal sediments offshore from rivers, which is needed to strengthen the interpretation of paleoreconstructions. Here, we analyze the palynological content of marine surface sediments along land–sea transects off the Mississippi and Atchafalaya river mouths in the northern Gulf of Mexico (GoM) and test three palynological indices which are often employed in the paleo-domain: (i) the abundance of cysts of heterotrophic and autotrophic dinoflagellates (dinocysts) as a tracer for primary productivity (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M1" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi>H</mi><mo>/</mo><mi>A</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="22pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="fd1af044779a4f8d4f100f2d4e57c845"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="jm-42-257-2023-ie00001.svg" width="22pt" height="14pt" src="jm-42-257-2023-ie00001.png"/></svg:svg></span></span> ratio) and (ii) the ratio between non-bisaccate pollen and bisaccates (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M2" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi>P</mi><mo>/</mo><mi>B</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="23pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="8a2e050f4b46caec36f7c0e135e62831"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="jm-42-257-2023-ie00002.svg" width="23pt" height="14pt" src="jm-42-257-2023-ie00002.png"/></svg:svg></span></span>) as well as (iii) the ratio between pollen (excluding bisaccates) and dinocysts (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M3" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi>P</mi><mo>/</mo><mi>D</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="23pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="1379e013a0c44dbff6a09132ca2d92e3"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="jm-42-257-2023-ie00003.svg" width="23pt" height="14pt" src="jm-42-257-2023-ie00003.png"/></svg:svg></span></span>), which are both tracers for river input and distance to the coast.</p> <p>Our results show that dinoflagellate cysts are most abundant on the shelf, where heterotrophic dinocyst species dominate coastal assemblages in reach of the river plume, while autotrophic taxa are more present in the oligotrophic open ocean. This is clearly reflected in decreasing <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M4" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi>H</mi><mo>/</mo><mi>A</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="22pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="467b9791936093d95cde6296afe1364b"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="jm-42-257-2023-ie00004.svg" width="22pt" height="14pt" src="jm-42-257-2023-ie00004.png"/></svg:svg></span></span> values further offshore. Individual dinocyst taxa also seem to inhabit specific niches along an onshore–offshore transect, linked to nutrient availability and proximity to the turbid river plume. The highest pollen concentrations are found close to the Mississippi river mouth and mostly represent a mixture of local coastal and upstream vegetation, whereas bisaccate pollen was most abundant further offshore of the Mississippi river. Multivariate redundancy analysis (RDA) performed on both pollen and dinocyst assemblages, a set of environmental parameters, and the three palynological ratios showed that net primary productivity was the most important variable influencing the dinocyst assemblages, likely as the result of nutrient input. Additionally, the RDA confirmed that the <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M5" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi>H</mi><mo>/</mo><mi>A</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="22pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="e612397d9f3df24db4dde93b9c975102"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="jm-42-257-2023-ie00005.svg" width="22pt" height="14pt" src="jm-42-257-2023-ie00005.png"/></svg:svg></span></span> ratio indeed seems to track primary productivity, while the <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M6" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi>P</mi><mo>/</mo><mi>B</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="23pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="67f79a7edf7ce2c0ba2f7c7208180fc0"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="jm-42-257-2023-ie00006.svg" width="23pt" height="14pt" src="jm-42-257-2023-ie00006.png"/></svg:svg></span></span> ratio results in a robust indicator for distance to the coast, and the <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M7" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi>P</mi><mo>/</mo><mi>D</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="23pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="141f3a48f865d92c4ef19785b57e6370"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="jm-42-257-2023-ie00007.svg" width="23pt" height="14pt" src="jm-42-257-2023-ie00007.png"/></svg:svg></span></span> ratio better reflects river input. Together, our data confirm and further specify the suitability of these three palynological ratios in river-dominated coastal margins as proxies for (past) marine productivity and distance to the coast and river.</p>