Spatial distribution of biogenic and lithogenic sediment components in İzmit Bay: Assessing eutrophication dynamics and anthropogenic pollution sources in a stratified coastal ecosystem

<span style="color: rgb(97, 97, 97); font-family: &quot;Open Sans&quot;, Helvetica, Arial, sans-serif; font-size: 14px;">In the present study, the spatial distribution and interrelationships of major biogenic and lithogenic components—biogenic silica (BSi), calcium carbonate (CaCO</span><span style="font-size: 10.5px; line-height: 0; position: relative; vertical-align: baseline; bottom: -0.25em; color: rgb(97, 97, 97); font-family: &quot;Open Sans&quot;, Helvetica, Arial, sans-serif;">3</span><span style="color: rgb(97, 97, 97); font-family: &quot;Open Sans&quot;, Helvetica, Arial, sans-serif; font-size: 14px;">), total organic matter (TOM), total organic carbon (TOC), lithogenic matter (LM), calcium (Ca), and phosphorus (P)—were examined in İzmit Bay surface sediments (0–4 cm). The research assessed their utility as proxies for tracing eutrophication and contamination in the heavily industrialized part of the Marmara Sea. Sediments from 56 stations were analysed using classical wet‑chemistry and combustion methods, and Ca and P concentrations were determined by ICP‑MS. Distributions were mapped using Kriging interpolation. Results indicate that complex hydrodynamics and intense anthropogenic pressures drive spatial partitioning. BSi, a proxy for diatom activity, reached its highest average concentration (12.5%) in the shallow Eastern Basin. Elevated BSi and TOC/TOM near wastewater discharge points signal nutrient (specifically TN and TP) enrichment, excessive primary production, and severe eutrophication. CaCO</span><span style="font-size: 10.5px; line-height: 0; position: relative; vertical-align: baseline; bottom: -0.25em; color: rgb(97, 97, 97); font-family: &quot;Open Sans&quot;, Helvetica, Arial, sans-serif;">3</span><span style="color: rgb(97, 97, 97); font-family: &quot;Open Sans&quot;, Helvetica, Arial, sans-serif; font-size: 14px;">&nbsp;peaked (16.0%) along the northern coast, linked to cement industry inputs and dolomite deposits. High TOC and TOM in deep, hypoxic/anoxic zones reflect mixed autochthonous (phytoplankton‑derived) and allochthonous (land‑based/industrial) organic‑matter inputs. The negative correlation between LM and biogenic components confirms the dilution effect of fluvial mud. Notably, the 61% increase in TOC levels in the surface sediments since 1987 indicates ongoing environmental deterioration in İzmit Bay. In conclusion, elevated BSi, TOM, TOC, and P values collectively signal nutrient‑driven eutrophication, while high CaCO</span><span style="font-size: 10.5px; line-height: 0; position: relative; vertical-align: baseline; bottom: -0.25em; color: rgb(97, 97, 97); font-family: &quot;Open Sans&quot;, Helvetica, Arial, sans-serif;">3</span><span style="color: rgb(97, 97, 97); font-family: &quot;Open Sans&quot;, Helvetica, Arial, sans-serif; font-size: 14px;">&nbsp;reflects industrial discharge. An immediate reduction of nutrient inputs is essential to mitigate eutrophication, chronic algal blooms, mucilage occurrence and prevent widespread hypoxia/anoxia in İzmit Bay.</span>