The concentration of dissolved oxygen, salinity, and sediment load are the most critical vector characteristics of estuary water. Salinity varies greatly by location, ranging from near zero at the tidal limit of tributary rivers to 3.4 percent at the estuary mouth. The salinity can differ greatly over time and seasons at any given stage, making it a harsh environment for species. Intertidal mudflats are notoriously difficult to colonise, and sediment often settles there. Since there are no points of attachment for algae, no vegetation-based habitat can be created. Sediment can clog a species' feeding and respiratory structures, and mudflat species have evolved unique adaptations to deal with this issue. Finally, variations in dissolved oxygen can cause problems for life forms. Nutrient-rich sediment from man-made sources may encourage primary production life cycles, potentially contributing to decay that depletes dissolved oxygen in the water, resulting in hypoxic or anoxic areas.