Ocean acidifcation caused by the increase in atmospheric C02 may adversely affect starfish and many other marine invertebrates. The starfish skeleton is made of calcite ossicles which dissolve under acid conditions. But it is difficult to predict how different species will cope with these changes – some may cope better than others.
I think this is a very difficult question. Warming sea surface temperatures due to global warming will allow the snake pipefish to breed more often and the offspring to develop faster, which will make population size increase. However, increases in pollution, including micro-plastics, and the loss of algal beds (their habitat along the shores) will make them less healthy which may reduce their reproductive rate and reducing their habitat will likely increase mortality. How all these factors interact with each other, and what general trend (positive or negative) they produce in end, we still don’t know.
That’s quite difficult to say. We are scavengers (we eat everything going but mostly find it easiest to sniff out dead things!) so we could be affected by overfishing. If humans remove too many fish from the shallow waters it could leave us less food to eat and our population is likely to decrease.
We’ve also recently learned that microplastics are found in the stomachs of deep-sea animals so all the pollution that is happen in the surface of the sea is affecting the deep sea too! This could also have a negative impact and reduce our population size.